MB-8855 Lower xdcrMaxConcurrentReps limit.

Change-Id: I6e0dd0e5327e67c01de132be5667139546b0deec
Reviewed-on: http://review.couchbase.org/28316
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>