fix data race in code for logging login attempts

logging of login attempts happens in the background, because we don't want to
block regular operation with disk since for such logging. however, when a line
is logged, we evaluate some attributes of a connection, notably the username.
but about when we do authentication, we change the username on a connection. so
we were reading and writing at the same time. this is now fixed by evaluating
the attributes before we pass off the logger to the goroutine.

found by the go race detector.

store/loginattempt_test.go

@@ -17,12 +17,19 @@ func TestLoginAttempt(t *testing.T) {
 	mox.MustLoadConfig(true, false)
 
 	xctx, xcancel := context.WithCancel(ctxbg)
+	defer xcancel() // Stop clearing of LoginAttempts.
 	err := Init(xctx)
 	tcheck(t, err, "store init")
-	// Stop the background LoginAttempt writer for synchronous tests.
-	xcancel()
-	<-writeLoginAttemptStopped
+	stopc := make(chan struct{})
+	writeLoginAttemptStop <- stopc
+	<-stopc
 	defer func() {
+		// Ensure Close() below finishes
+		go func() {
+			c := <-writeLoginAttemptStop
+			c <- struct{}{}
+		}()
+
 		err := Close()
 		tcheck(t, err, "store close")
 	}()