Before, when solving a solver, parsing sub objects of solver_process was done directly synchronously
in solver_workflow. And we had the case where you have parallel transactions solving a solver,
each of theses transactions see remaining solver not in solved state, but once all transaction are
finished, all solver are solved. This could lead to the case where solver_process is never moved
to succeeded. Instead of using serialize (which may lead to conflicts), just use activities in queue
with a serialization tag.

/reviewed-on nexedi/erp5!714

/reviewed-on nexedi/erp5!697

/reviewed-on nexedi/erp5!713

of Free Subscription Request module and FreeSubscription_viewFreeSubscriptionRequest
view of Free Subscription

Extend`FTPConnector.listFiles` to sort files based on some server-side computed attributes of files.

This uses [`listdir_attr`](http://docs.paramiko.org/en/2.4/api/sftp.html#paramiko.sftp_client.SFTPClient.listdir_attr) method of paramiko. `sort_on` parameter is directly attributes of [`paramiko.sftp_attr.SFTPAttributes`]( http://docs.paramiko.org/en/2.4/api/sftp.html#paramiko.sftp_attr.SFTPAttributes).

At the same time, adds a minimal tests, that will only run if `testSFTPConnection_SFTP_URL` environment variable is set to an URI of a running sftp server supporting password authentication - which means that in our current tests infrastructure it will not run.

/cc @vpelletier @aurel @rafael 

/reviewed-on nexedi/erp5!680

/reviewed-on !711

And not the string "None". I thought ZSQLMethods were supposed to complain
when receiving None for non-optional fields...

manage_afterClone clears workflow history, which is precisely what
"duplicate" is trying to avoid.
This fixes a regression introduced in
  commit 7e810d5a
  Author: Vincent Pelletier <vincent@nexedi.com>
  Date:   Tue Feb 20 17:06:46 2018 +0900

      CopySupport: Factorise clipboard-related methods and code paths.

The Diff view in new UI will be using IFrames to display the diff.

Also, add changes in tests:

erp5_ui_test: Update history test according to use of diff gadget
erp5_web_renderjs_ui_test: Update tests according to new Diff view for Historical Revisions
testXHTML: Add Historical Comparison View to the list of listbox with no selection name
testXHTML: Simplify test_html_file and include way to ignore single html file

Language modification is done on another page, accessible from many places for now.

The original need to touch this comment is that it mentions now-removed
Folder_reindexObjectList script. But also:
- it mentions multi-ZEO setups, which are now discouraged if favour of
  using NEO
- it mentions pre-conceived optimisations, which can be wrong and hence
  misleading (up-to-date measures are needed before deciding what to
  optimise)
- it mentions a number of activity nodes which is nowadays exceeded
  without seeing any obvious issue caused by activity table deadlocks
  (which are nevertheless still likely an issue, just not yet breaking
  scalability)

These are now superseded by recursiveReindexObject.

This method does not belong to Folder (nor Base, even as an alias), as
it recurses wihout any limit in a tree whose node count is unknown.
Such generic recursion should either be deferred (split in multiple
transactions, each processing a limited chunk of the tree) or not be
recursive (which of course depends on the need).
There is a single caller left (constructInstance) where the subtree size
is assumed to be reasonable, as it was constructed inside current
transaction. So essentially rename this method so it is only used in such
conditions.

isAncestryIndexable is a parent-recursive version of isIndexable: it
will return False if any parent is non-indeable. This simplifies
disabling indexation of an entire tree, from its root to its deepest
leaves.
isSubtreeIndexable allows preventing any subobject from being indexable,
without having to mark each individual subobject as non-indexable. This
is different from isAncestryIndexable, as it allows having an indexable
root, but no children indexable. This is especially useful for Trash Bins,
where the bin itself should be indexable (because there is no reason not to)
but its content must not be indexed, while being left as it was found in its
original location.

Should make Folder_reindexAll and most custom indexation methods obsolete.
Remaining valid reindexation methods are:
- reindexObject: for a single document, which may contain subdocuments
  which indexation is not necessary
- recursiveReindexobject: for any subtree of documents
- ERP5Site_reindexAll: for site-wide reindexations, as there is a semantic-
  dependent indexation order.

Also, uniformise and factorise spawning immediateReindexObject.

Also:
- testSupply: Drop check for the previous magic threshold.
  _recurseCallMethod takes care of it all now.
- testXMLMatrix: Let activities execute before changing cell id.
  This works only because recursiveReindexObject on the matrix spawns a
  single recursiveImmediateReindexObject activity on that context. Now,
  up to 1k immediateReindexObject activities (for the first 1k sub-objects)
  are spawned immediately, preventing their renaming immediately after
  commit.
  So let test wait for indexation before trying to rename.
- testERP5Security: More activities are now spawned immediately, adapt.

after_method_id is too aggressive a dependency.
after_path_and_method_id is marginally better, but is bad practice
nevertheless, as such activity dependencies remain very hard to introspect.
Tags allow more variations, and are hence easier to follow.
Also, add XXX comments when (recursive) reindexation is only added to
represent the dependency, without certainty that a reindexation is actually
needed per-se: maybe another place in the code chould just appropriately
tag its reindexation activities instead, saving at least the cost of
spawning one more activity set, and at most the cost of actually
reindexing these documents.

Also, drop isIndexable instance property, unmasking class property, which
already has the value we need.
Also, factorise catalog clearing by reusing ERP5Site_reindexAll's.

Allows having a more consistent API between this tool and all other
Nexedi-maintained tools.