journal: added a per-zone lock for journal access.

 - When reloading a server, there's a possibility of two readers reading
   the journal, but that should be fine. This is because the zone
structure is recreated during reloads, and so is the lock. Only events
can write to journal though, so there's no possibility of concurrent
read and write.
journal: added lock for journal flush
tests-extra: IXFR stress test