Port from ZODB 3.2.

Added new test checkSubtxnCommitDoesntGetInvalidations to
verify that a longstanding bug in subtransaction commit is
repaired.

Jim (Fulton) discovered this in ZODB 3.4's code, while implementing
savepoint/rollback.  Same bugs had been there at least since ZODB 3.1.

Also added news about the bug.