MB-11423: keep connection alive in CBRecoveryTask

Change-Id: I7eb1df24ac51de80f9f0257a01d4e5477aada42b
Reviewed-on: http://review.couchbase.org/38481
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>