imapclient: implement Client.Closed

[WhyNotHugo]

Implement a Client.Closed() to be used by clients to monitor when a connection is closed.

[WhyNotHugo]

I've implemented by particular case in some other way, but this sounds useful to have. I have no strong objections if you'd prefer to close this PR.

[emersion]

I was wondering about the use-case for this. I initially thought I needed something similar for alps, but then figured it wouldn't help much:

• The connection might be broken even when the reader goroutine is still alive, when a connection isn't explicitly closed by the remote and times out.
• Even if the reader goroutine could perfectly detect when a connection is broken, checking for the closed channel introduced in this PR before sending a command would be racy.
• A disconnected client only consumes a small amount of memory, its underlying FD is closed.

[WhyNotHugo]

checking for the closed channel introduced in this PR before sending a command would be racy.

The channel is not intended to be checked before sending a message, but to re-connect automatically when the connection terminates (i.e.: in the case of NOTIFY).

[WhyNotHugo]

Previously, the Notify.Overflow channel in #718 would also be closed when the connection dropped (a convenient side-effect). This allowed me to detect when a connection had been closed.

Having switched NOTIFY to use a UnilateralDataHandler for NOTIFYOVERFLOW events, I once again need this feature.

I don't think I have any other way of detecting when the connection has been closed. I'm only waiting for NOTIFY events, so never send any messages.

[emersion]

Hm, right, that's a good point. I think NOTIFY is special here: with IDLE, clients could call IdleCommand.Wait to figure out when the connection was broken. Since NOTIFY isn't a long-running command, there's no equivalent.