rename SERVER_BUSY to CONNECTION_REFUSED

[kazuho]

Please see the discussion on #3690 for the rationale, but in short, SERVER_BUSY is renamed as CONNECTION_REFUSED. A server might refuse a connection for various reasons. SERVER_BUSY is one specific reason. Having an error code for that specific case while not having an error code for generic case is awkward. Hence the rename.

Closes #3690.

[marten-seemann]

I'm not sure if this was an editorial PR. The previous text talked about a situation where a server was too busy to accept a new connection. The new text generalizes this to a connection refusal for any reason. That's a design change.

I also strongly disagree that this issue should close #3690. There's still on ongoing discussion if an additional error code should be introduced. I can't see how this is not a design issue.

[MikeBishop]

There was previously no requirement to send this error code under any particular circumstances nor any requirement to take action upon receipt of this error code, and therefore those requirements cannot have been changed. This modifies the prose around an error code the server could use to reject a connection attempt to make the recommendation more narrow.