mon: compact on startup earlier

Do this right after we open the store.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: close/reopen leveldb before/after fork

If we blindly fork() we kill off leveldb's internal background thread
and get stalls later.

Fixes: #4851
Signed-off-by: Sage Weil <sage@inktank.com>