don't overload replication manager with irrelevant config events
don't overload replication manager with irrelevant config events

Now subscribe handler only sends {buckets, _} config updates to
replication manager. Which is the only config change event that's
handled by it.

Another change is consuming of all outstanding {buckets, _} events, so
that sequence of rapid buckets config changes do not cause unneeded
and unnecessary CPU load.

Change-Id: I4e2ac7e9f6c7c985d10f1559bbf8afcf342d5e08
Reviewed-on: http://review.couchbase.org/10556
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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