res_pjsip_registrar: Improve performance on inbound handling.

This change removes a sorcery lookup for retrieving all
contacts at the end of the registration process by keeping
track of the contacts that are added/updated/deleted.

This ensures at the end of the process the container of
contacts we have is the current state.

Pool usage has also been reduced by allocating one for
usage throughout the handling of a REGISTER and resetting
it to a clean state. This ensures that in most cases
we allocate once and just reuse it.

ASTERISK-28001

Change-Id: I1a78b2d46f9a2045dbbff1a3fd6dba84b612b3cb