Make file and KV store distinction clear in public APIs.

The following APIs and structures are renamed as follows:

APIs:
fdb_get_dbinfo  --> fdb_get_file_info
fdb_get_seqnum --> fdb_get_kvs_seqnum

Structure:
fdb_handle  --> fdb_kvs_handle
fdb_info  --> fdb_file_info

Change-Id: Ia822aa1f61a087feec4e86f07cdc9fa855911191