redis-trib: allow support for mandatory options.
CLUSTER MEET: better error messages when address is invalid.

Fixes issue #1734.
Cluster: initial ability to import data from standalone instance.
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).