CBQE-1003: move common func in XDCRReplicationBaseTest

_async_modify_data and _async_update_delete_data duplicated

Change-Id: Id46633d7ab967afdea6de198a651d79f32694251
Reviewed-on: http://review.couchbase.org/24320
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>