udp: fix helpers mutex locking

[cspiel1]

The current solution of protecting the helpers list against processing while there is a helper added/deleted is not sufficient.

Adding a helper most likely is save.

Deleting a helper makes the list element le invalid, further the UDP helper uh becomes invalid and may not be called.

[cspiel1]

Currently there is a deadlock in retest:

• udp.c:199 the recvh of a helper calls indirectly
• `udp_send_helper() and then tries to lock the mutex at udp.c:879

edit:
We discussed also a two mutex solution. One for receive and one for send. This also would lead to a deadlock, because
a normal use case is that a send helper may invoke udp_send_helper(), like it is done in baresip-s bundle.c. This again would lead to a deadlock if the lock is hold during udp_send_internal().

[cspiel1]

It's not possible to make helper_destructor() and processing of the helpers list thread safe. The upper layer has to ensure that no helper is freed while the stream is still active.

In this case it has to be solved in baresip/baresip#3012