[Compaction] Add DB::GetColumnFamilyMetaData()

Summary:
Add DB::GetColumnFamilyMetaData(), which would be useful when only
information about one column family is needed (eg. after
OnFlushedCompleted(DB* db, std::string cf_name) is called.)

The implementation of GetDatabaseMetaData() is changed to
use GetColumnFamilyMetaData() internally.

Test Plan:
make compactor_test -j32
./compactor_test

make db_test -j32
export MetaData
./db_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22245