MB-100: adjust mem_quota in xperf lag config

Change-Id: Iba37f8cf42f0c50be0c2b8e1f2c25935f3f19da5
Reviewed-on: http://review.couchbase.org/24769
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>