Auto-enter slaveMode when SYNC from redis-cli

If someone asks for SYNC or PSYNC from redis-cli,
automatically enter slaveMode (as if they ran
redis-cli --slave) and continue printing the replication
stream until either they Ctrl-C or the master gets disconnected.
Merge pull request #1545 from mattsta/fix-redis-cli-sync

Deny SYNC and PSYNC in redis-cli
Sentinel test: added empty units to fill later.
Sentinel test: --pause-on-error option added.

Pause the test with running instances available for state inspection on
error.
Sentinel test: minor fixes to --pause-on-error.