mon: do not reopen MonitorDBStore during startup

mon: do not reopen MonitorDBStore during startup

level doesn't seem to like this when it races with an internal compaction
attempt (see below).  Instead, let the store get opened by the ceph_mon
caller, and pull a bit of the logic into the caller to make the flow a
little easier to follow.

    -2> 2013-06-25 17:49:25.184490 7f4d439f8780 10 needs_conversion
    -1> 2013-06-25 17:49:25.184495 7f4d4065c700  5 asok(0x13b1460) entry start
     0> 2013-06-25 17:49:25.316908 7f4d3fe5b700 -1 *** Caught signal (Segmentation fault) **
 in thread 7f4d3fe5b700

 ceph version 0.64-667-g089cba8 (089cba8fc0e8ae8aef9a3111cba7342ecd0f8314)
 1: ceph-mon() [0x649f0a]
 2: (()+0xfcb0) [0x7f4d435dccb0]
 3: (leveldb::Table::BlockReader(void*, leveldb::ReadOptions const&, leveldb::Slice const&)+0x154) [0x806e54]
 4: ceph-mon() [0x808840]
 5: ceph-mon() [0x808b39]
 6: ceph-mon() [0x806540]
 7: (leveldb::DBImpl::DoCompactionWork(leveldb::DBImpl::CompactionState*)+0xdd) [0x7f363d]
 8: (leveldb::DBImpl::BackgroundCompaction()+0x2c0) [0x7f4210]
 9: (leveldb::DBImpl::BackgroundCall()+0x68) [0x7f4cc8]
 10: ceph-mon() [0x80b3af]
 11: (()+0x7e9a) [0x7f4d435d4e9a]
 12: (clone()+0x6d) [0x7f4d4196bccd]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to interpret this.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit ea1f316e5de21487ae034a1aa929068ba23ac525)