Ruby: pass Packet->Req->contextId() to Ruby.

Ruby: pass Packet->Req->contextId() to Ruby.
It is useful for Ruby to understand from whence request packets came.
This has all request packets going into Ruby pass the contextId value, if
it exists.  This supplants the old libruby proc_id value passed around in
all the Messages, so I've also removed the unused unsigned proc_id; member
generated by SLICC for all Message types.