Distinguish server and client debug logs

[kayrus]

Currently the debug mode writes all the input and output into one stream:

go-smtp/conn.go

Lines 84 to 85 in 26eb481

	io.TeeReader(rwc.Reader, c.server.Debug),
	io.MultiWriter(rwc.Writer, c.server.Debug),

I'd like to have independent debug streams and alternatively skip or mask AUTH command details.

Probably it's also worth adding an error handler inside the WriteResponse method, e.g. ResponseLog but not in server, but connection scope to have an ability to specify the requestID/connectionID.

[kayrus]

@emersion something like this for sessions logging: https://gist.github.com/kayrus/860c1e9d522e382fcf0b6b8f420017b9

[kayrus]

Another use case is to catch stale connection events, when AUTH was done but the rest wasn't, and a connection was timed out.

[emersion]

Sorry, but I'll close this as out-of-scope. The DebugWriter functionality was never meant to be a way for library users to add more functionality, it's purely a debugging tool.