MEDIUM: connection: add definitions for dual polling mechanisms

MEDIUM: connection: add definitions for dual polling mechanisms

The conflicts we're facing with polling is that handshake handlers have
precedence over data handlers and may change the polling requirements
regardless of what is expected by the data layer. This causes issues
such as missed events.

The real need is to have three polling levels :
  - the "current" one, which is effective at any moment
  - the data one, which reflects what the data layer asks for
  - the sock one, which reflects what the socket layer asks for

Depending on whether a handshake is in progress or not, either one of the
last two will replace the current one, and the change will be propagated
to the lower layers.

At the moment, the shutdown status is not considered, and only handshakes
are used to decide which layer to chose. This will probably change.