udp: add a lock for the helpers list

[cspiel1]

relates to RX-thread: baresip/baresip#2445

The helpers list may be changed from main thread while the RX thread processes the helpers.

Edit: More concretely the bundle feature in baresip could add the bundle helper during the call, with UPDATE or re-INVITE.

[cspiel1]

list_unlink() should also be locked.

[cspiel1]

There is a dead lock currently in dtls_estab_handler() with selftest.

[cspiel1]

Solution: Maybe two different locks

• a lock for the helpers list which should not be locked while the helper callback is called
• a second lock that protects the helper itself to be deleted while the helper callback is called

Edit:
The implementation of the helper has to take care that it is not deleted while it is in the callback.

[cspiel1]

The new solution protects only changes of prev and next pointer. Should we use atomic?

Protection against helper deletion vs. processing concurrency has to be done in the upper layer.