pytalloc: Improve timer wrapper, and test it

pytalloc: Improve timer wrapper, and test it

Using Context.add_timer resulted in crashes due to missing type object
and bad reference handling.
Add a TeventTimer_Type struct, and introduce a clear ownership/lifetime model.

Add a "add_timer_offset" to allow adding timers from Python. (add_timer
requires passing struct timeval as a Python integer, which can't really
be done portably).

Add tests.

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jelmer Vernooij <jelmer@samba.org>