ackhandler: immediately acknowledge ECN-CE marked packets

[marten-seemann]

Fixes #4146.

Sending an ACK immediately when receiving an ECN-CE marked packet allows the sender to react to the marking faster. This is expected to be crucial for L4S to work properly, especially when used in conjunction with the ACK frequency extension.

[marten-seemann]

This is likely important for performance. When using L4S, we expect 2 CE-markings per RTT, so we want to avoid calls to Debugf when we're not actually outputting logs.

[sukunrt]

Do we need this? The Debugf implementation is doing the same check.

// Debugf logs something
func (l *defaultLogger) Debugf(format string, args ...interface{}) {
	if l.logLevel == LogLevelDebug {
		l.logMessage(format, args...)
	}
}

[marten-seemann]

Right. It's only important when passing arguments to the function, because that would allocate a slice.

[codecov]

Codecov Report

Merging #4147 (fc8180f) into master (a360354) will increase coverage by 0.10%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4147      +/-   ##
==========================================
+ Coverage   83.75%   83.84%   +0.10%     
==========================================
  Files         150      150              
  Lines       15411    15454      +43     
==========================================
+ Hits        12906    12957      +51     
+ Misses       2007     1998       -9     
- Partials      498      499       +1     

Files	Coverage Δ
internal/ackhandler/received_packet_tracker.go	82.93% <100.00%> (+0.28%)	⬆️

... and 3 files with indirect coverage changes