os/LevelDBStore: do compact_prefix() work asynchronously

We generally do not want to block while compacting a range of leveldb.
Push the blocking+waiting off to a separate thread.  (leveldb will do what
it can to avoid blocking internally; no reason for us to wait explicitly.)

This addresses part of #5176.

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