b=1502

b=1502
- Discard incredibly old requests (older than "obd_timeout" seconds) in
  ptlrpc_main without even trying to process them, because the client
  has already given up on us.  See the bug text for more details on why
  this is the right thing to do.
- Clean up deeply-nested ptlrpc_main()
- On Zach's good advice, I also fixed Portals to record when a packet
  arrives in useful microseconds instead of relatively useless cycles