PSYNC2: bugfixing pre release.

1. Master replication offset was cleared after switching configuration
to some other slave, since it was assumed you can't PSYNC after a
switch. Note the case anymore and when we successfully PSYNC we need to
have our offset untouched.

2. Secondary replication ID was not reset to "000..." pattern at
startup.

3. Master in error state replying -LOADING or other transient errors
forced the slave to discard the cached master and full resync. This is
now fixed.

4. Better logging of what's happening on failed PSYNCs.
