Fix issues with FAILOVER

Fix issues with FAILOVER
In particular it is possible when doing a multi-node failover
with a cascaded node for the result after the failover to
have nodes with a self subscription, a row in sl_subscribe
with the provider and receiver equal.

In fixing that issue we also discovered that slon was not
reloading the set origin's into memory following a FAILOVER
command.  This could mean that we were processing SYNC events
from a node which without realizing that node now is the origin
for a set.