chan_pjsip: Add 'pjsip show channelstats'

chan_pjsip:  Add 'pjsip show channelstats'

Added the ability to show channel statistics to chan_pjsip (cli_functions.c)

Moved the existing 'pjsip show channel(s)' functionality from
pjsip_configuration to cli_functions.c.  The stats needed chan_pjsip's
private header so it made sense to move the existing channel commands as well.

Now using stasis_cache_dump to get the channel snapshots rather than retrieving
all endpoints, then getting each one's channel snapshots.  Much more efficient.

Change-Id: I03b114522126d27434030b285bf6d531ddd79869