MB-9209: hibernate idle couch_file readers

During xdcr those processes are reading tons of binaries. And after
burst of activity tend to go idle for long time. It makes sense to
hibernate those folks in this state. Releasing unused
memory. Especially given that it's state is just two args which is
cheap to gc/hibernate.

Change-Id: I6793dd7fcd29457f711a871bc12f3b7e275b56ff
Reviewed-on: http://review.couchbase.org/30357
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>