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 converter
check.  This isn't particularly pretty...

    -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>