ITS#8958 rename ldap_pvt_thread_pool_pausecheck()
to ..._pausewait() since that's what it really does.
Add ..._pausequery() that only checks and doesn't wait.
ITS#8958 back-mdb: checkpoint online indexer
Save old/new indexmasks to allow processing to resume if slapd is
stopped and restarted. Save last entryID indexed to allow resume
after restart or pool pause.