Move auxiliary IO dispatcher tasks to the global IO manager

Each couchbase bucket has it own auxiliary IO dispatcher thread that is used for warmup, backfill tasks from disk, and background fetches for replications. We introduced a new Global IO Manager (GIO) as of 2.1.0 release, which deals with all kinds of disk IO operations across buckets. Therefore, it will be better to move auxiliary IO dispatcher tasks to the GIO for better multi-tenancy support, and depreciate the auxiliary IO dispatcher.