MB-100: add type in concurrency throttle

Concurrency throttle is a generic service and components may create
their own throttle processes, e.g., in XDCR, there are two types of
concurrency throttlesP init throttle and replication throttle. But
without type in throttle state, it would be hard to make enchancement
such as adaptive scheduling and differentiate traces in logs from
different throttles.

Change-Id: I075df7db1dfc7b0e48448e26fa3bd08ef401f97a
Reviewed-on: http://review.couchbase.org/30945
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>