chan_sip: Prevent deadlock when issuing "sip show channels"

sip_show_channels locks the dialogs container first then locks each
sip_pvt so it can spit out the details.  The rest of sip dialog
processing locks the sip_pvt first then locks the dialogs container
if it needs to.  Both lock in the order they need but deadlocks can
result.  To fix, sip_show_channels and sip_show_channelstats have
been converted to use an iterator rather than ao2_callback.  This way
the container is locked only while getting the next entry and is
unlocked when the callback is called.

ASTERISK-23013 #close

Change-Id: Id9980419909e811f89484950ed46ef117b9eb990