- due to a thinko in c170fe3e (ZODB: Provide ZODB4-wc2 which backports MVCC approach from ZODB5)
  4-wc2 revision was set on a ZEO(egg) section instead of ZEO(repository)
  section. As the result it was ZEO4 - not ZEO4-wc2 - that was installed.

  This was leading to hangs in tests with wcfs waiting for ZODB server to
  update it after a commit, but no notification was coming from ZEO at all.

  For the refrence: the reason ZEO4-wc2 exists at all is the backport patches
  that teach ZEO to send invalidations even on empty transactions
  (nexedi/ZEO@bf80d23d), and to include
  just-created objects into invalidations as well (nexedi/ZEO@f2fae122).
  Those changes are needed for WCFS to be always notificated on every
  transaction and with full set of changed objects.

- there was a typo: it was `revisions` instead of `revision`.

-> Fix both issues. Now it is ZEO4-wc2 that is installed for real when `ZODB.major = 4-wc2`.

ZEO 5.2.3 fixes several client concurrency bugs that could result in
data corruption. See changelog in https://pypi.org/project/ZEO/5.2.3/
for details.

trollius 2.2.post1 -> 2.2.1 update fixes "bad file descriptor" problem for ZEO5.
See https://github.com/zopefoundation/ZEO/issues/176 for details.

futures 3.2.0 -> 3.3.0 update brings in more efficient use of threads in
ThreadPoolExecutor. See https://github.com/agronholm/pythonfutures/blob/master/CHANGES.rst#330
for details.

To always call loadBefore instead of load.
Wendelin.core 2 needs this - see nexedi/ZODB@8e7eab33 for details.

There is also ZEO4-wc2 which contains backports of ZEO5 patches that are
needed for wendelin.core 2 to work correctly.

Wendelin.core 2 will work with ZODB4-wc2 or ZODB5.
Wendelin.core 2 will not work with just ZODB4.

Factor-out things that provide components for ZODB stack from software/neoppod/
into component/ZODB/.

Move versions for eggs that component/{ZEO,pygolang,zodbtools,pytest}
depend on out of component/ and into stack/slapos.cfg

Leave version of component <X> inside component/<X>.

I was asked to do so:

nexedi/slapos!839 (comment 119170)

We already patch ZEO4 with TCP_NODELAY patch (see 5cf4cf1f "ERP5:
enable TCP_NODELAY for ZEO") and we will need to backport more patches
to ZEO4 branch for wendelin.core 2 to work correctly.

It's not only software/neoppod which uses ZEO, and it is not convenient for
all other software-releases to inherit from neoppod to use correct
version and build of ZEO egg. For this reason factor out details of ZEO
egg building into component/ZEO and let users use ${ZEO:egg} where ZEO
is needed. This way ZEO will be correctly installed for all users.

This patch should be a non-functional change. We switch to
nexedi/ZEO@5114f909 revision which corresponds to ZEO 4.3.1 +
TCP_NODELAY.patch

Adding other patches to ZEO4 needed by wendelin.core 2 will be done as a
separate step.