redis-trib fix: use MIGRATE REPLACE when fixing slots.

This fixes issue #1765.
redis-trib fix improved: move keys from N nodes to owner.
Fix an error in redis-trib where we always talk with same node.

While iterating the list of nodes we want to set the slot as stable in
the current node, not always in the first node of the list.
Test: dump.tcl fixed for RESTORE new error msg.
Test: AOF test false positive when running in slow hosts.

The bug was triggered by running the test with Valgrind (which is a lot
slower and more sensible to timing issues) after the recent changes
that made Redis more promptly able to reply with the -LOADING error.
Cluster: slave validity factor is now user configurable.

Check the commit changes in the example redis.conf for more information.
Tag every log line with role.

Every log contains, just after the pid, a single character that provides
information about the role of an instance:

S - Slave
M - Master
C - Writing child
X - Sentinel
redisLogFromHandler() format changed to match new logs format.
Document log format change in 3.0 Changelog.