add rocksdb support

This patch added rocksdb support for ceph, enabled rocksdb for omap directory.   Rocksdb source code can be get from link. To use use rocksdb, C++11 standard should be enabled, gcc version >= 4.7 is required to get C++11 support. Rocksdb can be installed with instructions described in the INSTALL.md file,  and  rocksdb header files (include/rocksdb/*) and library (librocksdb.so*) need to be copied to corresponding directories.
To enable rocksdb, add "--with-librocksdb" option to configure

Signed-off-by: xinxin shu <xinxin.shu@intel.com>