[bp] note that vbuckets of vbm sup child vbucket ids are sorted
[bp] note that vbuckets of vbm sup child vbucket ids are sorted

We already relied on vbuckets field being sorted.
ns_vbm_sup::set_nodes_replicas in fact heavily relied on it.

This change documents this assumption and fixes ns_vbm_sup:add_replica
that did not maintain vbuckets sortedness.

Few optimizations enabled by this sortedness are also implemented.

Originally reviewed on http://review.couchbase.org/12398

Change-Id: Id7f176e6a80531754fe35ff86fd51f9985098340
Reviewed-on: http://review.couchbase.org/13956
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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