Change processReadEvent to processURCEvent.... (more in Desc)

Restructure the URC parsing for UULOC/UUSIMSTAT/UUPSDA/UUHTTPCR/UUPING to match UUSORD/UUSORF/UUSOLI/UUSOCL.
Add the missing URCs to pruneBacklog.
Change poll so it also uses processURCEvent - avoiding a whole bunch of code duplication.
Comment the original URC parsing in poll. Deleting it soon - after a little more testing.
Add extra debug messages.

Author: PaulZC