Fixup LSM drop:

* If a bloom drop succeeded but the chunk drop failed, we could skip the
  metadata update. Which would lead to a confused LSM tree after restart.
* Handle the case where an LSM tree is missing obsolete files more gracefully.
Not that it's a big deal, but do we need an exclusive lock here?  It's probably safer to be sure there is only a single thread updating the metadata, if nothing else?