Reduce log spam from closed client connections.

Reduce log spam from closed client connections.

Added a bunch of tests for keepalive functionality and fixed two cases
where we'd log an exception when the client was gone.  ECONNRESET
errors in IOStream reads now just close the connection instead of
logging an error (the exception information is still available on
stream.error in the close callback for apps that want it).
HTTPConnection now also checks for a closed connection and cleans up
instead of logging an error.

IOStream now raises a new exception class StreamClosedError instead of
IOError.