When a particular AvahiWatch is being dispatched, it cannot
be freed because `watch_callback()` accesses it after
the callback.
Introduce a member (`dispatching`) to coordinate
the deferred freeing in that case.
The timeouts are not affected because the `AvahiTimeout`
object is not accessed after the callback is called.
Retrieve the pointer returned by `calloc()` and free that
instead of freeing the pointer to a member. This has worked
so far because as of yet `api` is the first member of the struct.
