MB-5307: avoid infinite loops in auto-compaction daemon
MB-5307: avoid infinite loops in auto-compaction daemon

If node has too few vbuckets trying to pick enough distinct vbuckets
cannot possibly work. Fix is to defer to simpler "I don't care too much
to be exact" way.

Change-Id: Ibaf5d6a8ae34497ef2769711c4802523b3f09335
Reviewed-on: http://review.couchbase.org/16546
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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