RESTORE: reply with -BUSYKEY special error code.

The error when the target key is busy was a generic one, while it makes
sense to be able to distinguish between the target key busy error and
the others easily.
redis-trib.rb: MIGRATE hardcoded timeout set to 15 sec.

Will be configurable / adaptive at some point but let's start with a
saner value compared to 1 sec which is not a good idea for big data
structures stored into a single key.
redis-trib import: trap MIGRATE errors.
Sentinel: Add "dir /tmp" directive in example sentinel.conf.
redis-trib create: use CONFIG SET-CONFIG-EPOCH before joining the cluster.

This way there is no need for the conflict resolution algo to be used in
order to start with a cluster where each node has a different
configEpoch.
fixed possible buffer overflow error
Fixed possible buffer overflow bug if RDB file is corrupted.

(Note: commit message modified by @antirez for clarity).
Cluster: bypass data_age check for manual failovers.

Automatic failovers only happen in Redis Cluster if the slave trying to
be elected was disconnected from its master for no more than 10 times
the node-timeout value. However there should be no such a check for
manual failovers, since these are initiated by the sysadmin that, in
theory, knows what she is doing when a slave is selected to be promoted.
Cluster: forced failover implemented.

Using CLUSTER FAILOVER FORCE it is now possible to failover a master in
a forced way, which means:

1) No check to understand if the master is up is performed.
2) No data age of the slave is checked. Evan a slave with very old data
   can manually failover a master in this way.
3) No chat with the master is attempted to reach its replication offset:
   the master can just be down.
Cluster: fixed data_age computation / check integer overflow.