osd: allow log trimming during recovery

We rely on min_last_complete_ondisk being accurate; as long as we have logs
from that point, the current replicas will be able to safely recovery via
their logs.  Bump the target size, just as we do with degraded.

Signed-off-by: Sage Weil <sage@inktank.com>
DNM: FileJournal: add committed_up_to to header and check in read_entry

header_t::committed_up_to provides a lower bound for safetly committed
journal entries.  If read_entry fails prior to committed_up_to, we
know we have a corrupt jorunal entry.  This patch also causes
the header to be written on every journal write...should not
go into master.

Signed-off-by: Samuel Just <sam.just@inktank.com>
fix decode
fix replay
force header write less often
no forced header update