common/options: enable multiple rocksdb compaction threads for filestore

common/options: enable multiple rocksdb compaction threads for filestore

One of the major benefits of rocksdb over leveldb is multithreaded
compaction. The default of 1 thread does not provide much benefit, and
is insufficient for heavy rgw workloads.

For high-write and delete omap workloads I've seen up to 8 compaction
threads be used.  There's little overhead to having a higher max than
are needed, so set the default to 8.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>
(cherry picked from commit 023fa810aa6b3af305e9027e3f717e54d1bb2712)

Conflicts:
    src/common/options.cc (trivial)