Support balancing across a subset of nodes
Support balancing across a subset of nodes

mem3_balance implicitly assumed the set of nodes over which the DB is
hosted is expanding.  We need to make a couple of small changes in the
case of cluster contraction.

BugzID: 20742

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb