CBQE-1079: move template string decoder to worker

Change-Id: Ib024247c68b02b0e0d82f9817d4fe1d93ff7ef5f
Reviewed-on: http://review.couchbase.org/24979
Tested-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>