Suggest moves from all donor nodes in parallel

Previously the generator would suggest all moves from the first node
before moving onto the second one.  In the case where the quantum of
jobs is much smaller than the number of moves per node this results in
the other donors being neglected for long periods.

BugzID: 24612
Allow for rebalancing "special" DBs

For example, _replicator or _users.

BugzID: 24612