Implement METADATA command to update client metadata

Implement METADATA command to update client metadata

The METADATA command can be used by other servers to update "metadata"
of registered clients, like the client info text ("real name"), user
name, and hostname:

  :<prefix> METADATA <target> <key> :<value>

It is distributed in the network, unknown <key> names are silently ignored
and passed on, too. This allows for further extensions.