MB-20940: [12/n] Tap Removal - Remove CheckpointState Class
MB-20940: [12/n] Tap Removal - Remove CheckpointState Class

As part of Tap Removal, this patch removes the unused CheckpointState
class.  In addition it removes the proto_checkpoint_state enum.

Change-Id: I7632e8cc1a5992729d1b490511729ddd684608b0
Reviewed-on: http://review.couchbase.org/81087
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine