recovery: add special pull-logic for persistent databases

recovery: add special pull-logic for persistent databases

The decision mechanism which records of a persistent db
are to be pulled into the recdb during recovery is now
as follows:

* Usually a record with the higher rsn than that already
  stored is taken. (Just as for normal tdbs.)

* If a transaction is running on some node, then those
  nodes copies of all records are taken and are not
  overwritten later by other nodes' copies.

In order to keep track of whether a record's copy was obtained
from a node with a transaction running, the recovery mechanism
misuses the ctdb tdb header field 'lacount' in the recdb.
It is cleared later when pushing out the recdb database to the
other nodes.

This way, an incomplete transaction is not spoiled when
a recovery interrupts and the replay should usually succeed
(possibly after a few retries).

Michael