CBD-824: add XDCR_TIMING prefix

Change-Id: I14f9e4bfc932bdd708f1826df2e549135b7e0ef4
Reviewed-on: http://review.couchbase.org/24453
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>