Add replicaof command flag

[ut0mt8]

as discussed on slack for starting replication from now we need to interact with the redis protocol to set the master.
for auto starting replication we need something like --slaveof= flag.

(even if some ugly hack with systemd post start is possible)

[romange]

Lets make it --replicaof=foo:port

[royjacobson]

Lets make it --replicaof=foo:port

Just have to make sure we parse IPv6 addresses correctly (see https://softwareengineering.stackexchange.com/questions/311413/how-should-we-represent-an-ipv6-address-with-port-number-in-text)

[Kelvinyu1117]

I can help to implement this task.
What kind of argument format do we want to support? I can think of the followings:

• --replicaof=<ipv4>:port
• --replicaof=[<ipv6>]:port

[ut0mt8]

Ideally I think the both seems needed 👍

[adiholden]

@talbii Note: we will need to support setting this flag from config set command

[romange]

@adiholden it's not clear what's the expected behavior for this. Lets first check internally if it's really required.

[talbii]

Sounds completely reasonable, also shouldn't be a problem to implement in code. I'll do this tomorrow.

Might be a bit problematic since REPLICAOF NO ONE should modify this config, but we'll see 😸