Prevent spurious contextCSN generation
and ignore consumers when we have no contextCSN yet.
But make sure to propagate valid contextCSN updates to
accesslog/syncprov for delta consumers.

Windows mutexes are recursive so the trylock test will succeed
even though the mutex is already locked. Just compare be pointers.

The tree height can also increase during rebalance, not just shrink.
This can happen if update_key needs to split a parent branch page.

extended ops

Basic ops except abandon and unbind; since they get no reply
it's not important for the caller to know their msgID.

Schema parsing ignores some slapd.conf rules

In 2.4 syncrepl was changed to take its TLS defaults from the slapd
configuration (commit 1cc1f9b1) and the man page was updated to document
that (commit 27dd7a53). This updates the admin guide to match.

We have never documented our use of assert, so can't expect
builders to do the right thing.
This reverts commit 55dd4d32.

Not a bug.
This reverts commit 6fe51a9a.

config_fp_parse_line processes backslash escapes. When existing rewrite
rules were reloaded while inserting a new rule, this caused backslashes
to be lost from every rule except the most recently inserted one.
config_parse_ldif performs similar splitting, but leaves backslashes
alone.

Only usable if backend supports txns