http_server: consolidate fd/timer mangling into functions

Instead of open coding http_connection related I/O callback setup in
various parts of the code, create two functions

  - http_connection_start_expecting_requests()
  - http_connection_stop_expecting_requests()

The name should explain it all, one is used to enable the input callback,
the other suspends it.


Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
http_test: properly close http_connection at the end