Similarly to how it is done with e.g. http:// and https:// - if neos://
is given TLS usage is forced and ca/cert/key must be there either in the
URI itself, or in $NEO_CA, $NEO_CERT and $NEO_KEY environment variables
mimicking the way how e.g. for https:// TLS credentials are taken from
host environment, not from the uri.

The latter might be usability convenience, but is also useful for WCFS
which needs to be able to remove secrets from uri on zurl normalization.

Please see discussion at !18 (comment 184439)
for details.

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

(cherry-picked from commit bc3e38ea)

Because list of masters and cluster name must be already present in
netloc and path. Previously e.g.

	neo://db@α,β,γ?master_nodes=a,b,c"

would mean to use master nodes {a,b,c} not {α,β,γ}. Now it is treated as
invalid URI to remove ambiguity. Same for cluster name.

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

(cherry-picked from commit 22ccebd6)

zodburi[1] provides a way to open ZODB storages by URL/URI. It already
has support for file:// zeo:// zconfig:// memory:// etc schemes out of
the box and third-party-to-ZODB storages can add support for their
schemes via providing zodburi.resolvers entrypoint.

For example relstorage and newtdb do this.

Let's also teach NEO to open itself via neo:// URI schema.

[1] http://docs.pylonsproject.org/projects/zodburi
[2] https://github.com/zodb/relstorage/blob/2.1a1-15-g68c8cf1/relstorage/zodburi_resolver.py
[3] https://github.com/newtdb/db/blob/0.5.2-1-gbd36e90/src/newt/db/zodburi.py