Add support for taking snapshot of a column family and creating column family from a given CF snapshot.

When building a distributed key value store on top of rocksdb a useful feature is to be able to migrate a column family from one node to another. For this it will be useful to have APIs to
a. Generate a snapshot of a column family. This could be implemented by flushing the memtable and giving handles to all the sstables of the column family with their metadata information.
b. Create a column family from a snapshot. The snapshot can be transferred by the caller by copying the files and the metadata information and then it can call this API to create a column family with identical state as the sender.

Pls let us know if something for above already exists or can be extended or if anyone is planning to work on this?