pkg/log: add Clear to stop debug logging (#3372)

When reloading we need to disable debug output when the debug plugin is
removed from the config file. Add a `Clear` function to pkg/log and use
it in the server server.

Add test case in pkg/log, for actuall check I manually checked the
output by sprinkling some debug statements in the startup and checking
with sending SIGUSR1.

Also clear up the comments in pkg/log to remove the text about time
stamping.

Fixes: #3035

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/log/log_test.go

@@ -23,6 +23,13 @@ func TestDebug(t *testing.T) {
 	if x := f.String(); !strings.Contains(x, debug+"debug") {
 		t.Errorf("Expected debug log to be %s, got %s", debug+"debug", x)
 	}
+	f.Reset()
+
+	D.Clear()
+	Debug("debug")
+	if x := f.String(); x != "" {
+		t.Errorf("Expected no debug logs, got %s", x)
+	}
 }
 
 func TestDebugx(t *testing.T) {