MB-5602: consider buckets' servers list when computing down nodes
MB-5602: consider buckets' servers list when computing down nodes

Previous code assumed that all nodes need to have all buckets, but
that's not the case. Particularly, for partially rebalanced in or
partially rebalanced out nodes.

Fix is, naturally, to look exactly which buckets are expected to be up
and running on each node.

Change-Id: Iec3630a86fc92f213c32cf84f461388e4f5004f1
Reviewed-on: http://review.couchbase.org/17372
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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