MB-5307 Fix remote_clusters_info garbage collection.
MB-5307 Fix remote_clusters_info garbage collection.

Particularly keep 'clusters' list and 'buckets' list coherent with
what actually cached. Also don't forget to remove cluster cache entry
when it's removed from config.

Change-Id: I1eeaf9074f586d57a25accec5ead457127ee2ab0
Reviewed-on: http://review.couchbase.org/18953
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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