contrib: bump pjsip version to 2.10

also remove what has been upstreamed.
also, logic of how upstream has implements dealing with pending packets
on the socket is different now and therefore needs testing.

minor change:
- patches are now done in order.
- all configs are moved to config_site.h; TODO: also move HOSTVARS to use.mak
- enable epoll on linux

Change-Id: I87b9389a845d4c4a41fc9e70c3a94a0a990ca83d