ctdb-recoverd: Use rec->pnn everywhere

ctdb-recoverd: Use rec->pnn everywhere

This is currently referenced in a number of inconsistent
ways, including:

* pnn
* rec->ctdb->pnn
* ctdb->pnn
* ctdb_get_pnn(ctdb)
* ctdb_get_pnn(rec->ctdb)

The first of these always requires some thought about the context - is
this the node PNN or some other PNN (e.g. argument to function)?

rec->pnn is now always used when referring to the recovery daemon's
PNN.

Doing this also reduces reliance on struct ctdb_context internals.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>