redis-cli: add connect master command when we are connecting replica and we want to connect master

[huangzhw]

This PR adds connect master and connect replica to redis-cli to accommodate connect <host> <port>.

Sometimes I want to connect to master, so I connect to one node, execute
INFO replication to know whether this node is master.
If not, I execute connect master_host master_port.

I think it's useful to add connect master in redis-cli to automate this.

127.0.0.1:6380> connect master
127.0.0.1:6381> connect master
I'm already master

[oranagra]

seems useful to me.

[huangzhw]

Updated. A little concern about why roleCommand uses addReplyBulkLongLong to reply port.

[oranagra]

@redis/core-team please approve or comment.

[itamarhaber]

LGTM

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