res_pjsip/pjsip_transport_management.c: Fix deadlock with transport keep alive.

res_pjsip/pjsip_transport_management.c: Fix deadlock with transport keep alive.

Using the keep_alive_interval option can result in a deadlock between the
pjproject transport manager group lock and the monitored transports ao2
container lock.  The pjproject transport manager group lock has to be
superior in the locking order to the monitored transports ao2 container
lock because of pjproject callbacks called when already holding the group
lock.  The lock inversion happens when Asterisk attempts to send a keep
alive packet over the reliable transports.

* Made keepalive_transport_thread() iterate over the monitored transports
container rather than use the ao2_callback() method.  This avoids holding
the container lock when sending the keep alive packet.

ASTERISK-26686

Change-Id: I5d5392a52e698bbe41a93f7d8e92bf0e61fe3951