mon: fork early to avoid leveldb static env state

leveldb has static state that prevents it from recreating its worker thread
after our fork(), even when we close and reopen the database (tsk tsk!).
Avoid this by forking early, before we touch leveldb.

Hide the details in a Preforker class.  This is modeled after what
ceph-fuse already does; we should convert it later.

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