manager: rework the timestamps logic, so that they are an enum-index array

This makes things quite a bit more systematic I think, as we can
systematically operate on all timestamps, for example for the purpose of
serialization/deserialization.

This rework doesn't necessarily make things shorter in the individual
lines, but it does reduce the line count a bit.

(This is useful particularly when we want to add additional timestamps,
for example to solve #7023)

src/core/timer.c

@@ -419,7 +419,7 @@ static void timer_enter_waiting(Timer *t, bool initial) {
                                  * our own startup. */
                                 /* fall through */
                         case TIMER_STARTUP:
-                                base = UNIT(t)->manager->userspace_timestamp.monotonic;
+                                base = UNIT(t)->manager->timestamps[MANAGER_TIMESTAMP_USERSPACE].monotonic;
                                 break;
 
                         case TIMER_UNIT_ACTIVE: