kvdb-rocksdb: support the rocksdb/multi-threaded-cf feature

[cchudant]

The rocksdb crate supports a feature called multi-threaded-cf, which wraps column family descriptors into RwLocks. and change some types around.
Enabling it in a downstream app makes kvdb-rocksdb then fail to compile.

Questions:

• should I add a compile test to the CI to make sure future changes compile under that rocksdb flag?