As explained in https://github.com/zopefoundation/ZEO/issues/209 and
in recent patch titled "Add tests that demonstrates data corruption when
ZEO5 client is served by ZEO4 server" there is possibility of data
corruption when ZEO5 client loads data from ZEO4 server.

A fix for this is not trivial and would have to forward-port
load-tracking in client from ZEO4 to ZEO5. However, as discussed in
https://github.com/zopefoundation/ZEO/issues/209, we believe that noone
is actually using ZEO5.client-ZEO4.server configuration. Thus, given
that it was already planned to drop ZEO4 support soon, it was decided to
drop support for ZEO4 server instead of fixing it.

In this patch:

- we remove support for testing against ZEO4 server, including removing
  vendored ZEO4 copy.

- remove support for on-client methods that only ZEO4 server would call.
  This includes the sole method `serialnos`.

- remove support for connecting to any server besides one that
  interoperates with protocol '5'. ZEO4 used protocol '4'. It is
  explicitly tested by new test that updated ZEO5 client rejects
  connecting to a server that speaks protocol '4'.

- we do _not_ remove verify_invalidation_queue added by Jim in 2016 via
  5ba506e7 (Fixed a bug handling ZEO4 invalidations during cache
  verification) with the following message:

	ZEO4 servers can send invalidations out of order with
	``getInvalidations`` results, presumably because ``getInvalidations``
	didn't get the commit lock.  ZEO4 clients worked around this (maybe
	not directly) by queuing invalidations during cache verification.
	ZEO5 servers don't send invalidations out of order with
	``getInvalidations`` calls and the ZEO5 client didn't need an
	invalidation queue, except they do need one to work correctly with
	ZEO4 servers. :/

  This feature, even-though it is commented as being ZEO4-only, looks
  too risky to be removed in stable branch, especially taking into
  account that in https://github.com/zopefoundation/ZEO/pull/195
  @d-maurer instead of removing, preserved this queue in a similar form:

  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L90
  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L277-L280
  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L630-L641

  I believe it is better be safe than sorry.