Refactor: Rename assoc to Assoc and use constructor
Refactor: Rename assoc to Assoc and use constructor

By using a constructor we may use the new operator which allows
us to replace cb_mutex with std::mutex

Change-Id: Ie982c1e0b8ff5df76148420427c17adcf9817a38
Reviewed-on: http://review.couchbase.org/80565
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

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