Leverage the fact that there is never more than one socket active to
simplify the event loop's I/O processing. This removes the need to
look up active connections. Also removes the need to query the return
values from select since there is only one socket to process. This
change improves RPC throughput under the simulator by up to 20%
As part of this change I've clarified the code's use of the pyngus
connection by renaming the connection property to something a bit more
specific.
Change-Id: If7c020bb0bd96490af78bc06659db10073b02417
41e23c2436