OSD: batch trim for pg_log

For now, when the the number of pg_log entries is more than osd_min_pg_log_entries, at most time ceph will trim an entry at a time. Batching trim for speeding up.

Signed-off-by: Xinze Chi <xmdxcxz@gmail.com>
How about 'osd_pg_log_trim_min'?  Closer to the paxos trim min/max options.