ZODB: v↑ ZODB5 (5.6.0 -> 5.8.0)

Going from ZODB 5.6.0 to 5.8.0:

- removes leftovers of pre-MVCC read conflicts,
- relaxes assertion in tests that relate to NEO,
- brings up interface requirements in line with Wendelin.core 2 and WCFS,
- optimizes IO in fsrefs,
- fixes fsdump to match fsstats,
- provides new tests that exercise potential concurrency issues in
  storage implementations more thoroughly,
- provides other fixes and improvements.

Please see https://github.com/zopefoundation/ZODB/compare/5.6.0...5.8.0 and
https://pypi.org/project/ZODB/5.8.0/ for details.

component/ZODB/buildout.cfg

@@ -58,7 +58,7 @@ egg-versions =
 [ZODB5]
 <= _ZODB
 egg-versions =
-  ZODB = 5.6.0
+  ZODB = 5.8.0
   transaction = 2.4.0