jsonrpc: Fix Python implementation of inactivity logic.

When a JSON-RPC session receives bytes, or when it successfully sends
queued bytes, then it should count that as activity.  However, the code
here was reversed, in that it used the wrong check in each place.  That is,
when it tried to receive data, it would check whether data had just been
sent, and when it tried to send data, it would check whether data had just
been received.  Neither one makes sense and doesn't work.

Bug #13214.
Reported-by: Luca Giraudo <lgiraudo@nicira.com>
CC: James Schmidt <jschmidt@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>