osd: increase default leveldb write buffer, cache size

The FileStore's leveldb currently uses libleveldb's defaults for cache and
write buffer size, which are both 4 MB. Increase the cache size to 128MB and
the write buffer to 8MB.

Tested-by: Dmitry Smirnov <onlyjob@member.fsf.org>
Signed-off-by: Sage Weil <sage@inktank.com>