vacuum: correctly send TRY_DELETE_RECORDS ctrl to all active nodes
Originally, the control was sent to all records in the vnn_map, but
there was something still missing here:
When a node can not become lmaster (via CTDB_CAPABILITY_LMASTER=no)
then it will not be part of the vnn_map. So such a node would
be active but never receive the TRY_DELETE_RECORDS control from a
vacuuming run.
This is fixed in this change by correctly building the list of
active nodes first in the same way that the recovery process does it.
(This used to be ctdb commit
49247df4a47a8a107fa7dd7b187e69e243e6bdbe)