MT#4949 Use pbx in call flow graph.
MT#4229 Still log the error causing a 500.
MT#4955 Use cseq_method for subscriber captures.
MT#4955 Introduce customer default sound sets.

If a new contract sound set is marked as contract_default, use this
for every existing subscriber in contract_sound_set prefs if no
other set is specified yet.
If a new subscriber is created and such a sound set exists, also
use this one.

This prevents having to go over each single subscriber to assign
the contract_sound_set preference.