get a stable rocksdb (3.0)

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
add --with-librocksdb-static configure options

with this options, ceph will build rocksdb from submodule

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
add support for rocksdb as data store

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
rocksdb backend optimization

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
Merge pull request #2044 from xinxinsh/wip-rocksdb

Wip rocksdb

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>