Fix broken rdbWriteRaw() return value check in rdb.c.

Thanks to @PhoneLi for reporting.
Cluster: refactoring of slots update code and more.

The commit also introduces detection of nodes publishing not updated
configuration. More work in progress to send an UPDATE packet to inform
of the config change.
Cluster: UPDATE msg data structure and sending function.
Cluster: slots update refactored + UPDATE msg processing.

Now there is a function that handles the update of the local slot
configuration every time we have some new info about a node and its set
of served slots and configEpoch.

Moreoever the UPDATE packets are now processed when received (it was a
work in progress in the previous commit).
Cluster: replace hardcoded 4096 for bus msg len with sizeof().
Cluster: send a single UPDATE packet for now.
Cluster: removed not needed newline at end of redisLog() msg.