Set default value max_concurrent_reps_per_doc to 8
Set default value max_concurrent_reps_per_doc to 8

This seems to be a good default value. Anything higher than this is
causing performance to suffer in the general case.

Change-Id: I8bae05b141e7400b5877140baae0f26f669b2ad6
Reviewed-on: http://review.couchbase.org/14374
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Srinivas Vadlamani <4036f57732a648e71da3ac2c829c8239a16a4c5d@couchbase.com>

nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,daverigby/ns_server,vzasade/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,daverigby/ns_server,daverigby/ns_server,nimishzynga/ns_server,membase/ns_server,membase/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,membase/ns_server,pavel-blagodov/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,mschoch/ns_server