Make sure MetricsHandler doesn't crash upon multiple concurrent writes

[tadzik]

With heavy traffic, concurrent usage of TrackLoginState() and similar
could result in fatal error: concurrent map read and map write,
crashing the whole process.

The effectiveness of the fix can be observed with this test, which I'm not submitting because of it being an ugly hack:

package main

import (
	"testing"
	"time"

	log "maunium.net/go/maulogger/v2"
)

func TestConcurrentMetricsUpdates(t *testing.T) {
	sut := NewMetricsHandler("0.0.0.0:9876", log.Create(), nil, nil)
	go sut.Start()

	done := false

	go func() {
		for !done {
			go sut.TrackConnectionState("foo@bar.baz", true)
			go sut.TrackConnectionState("foo@bar.baz", false)
		}
	}()

	time.Sleep(5 * time.Second)
	sut.Stop()
	done = true
}

This explodes with the aforementioned message before the patch, survives and passes after.

I had another attempt at this using channels for synchronizing all these writes, but it was a PITA to get right (processing them only after Start() while closing them after Stop() while avoiding writing to closed channels...) that I figured I'd rather keep it simple, especially since the operations themselves are so trivial that the mutex is unlikely to cause any noticable issues anyway.

[tulir]

Looks sensible, thanks!