refactor: check for closed connection in wait functions

refactor: check for closed connection in wait functions

Catch and display errors in a more homogeneous way: if a wait function
finds a connection closed it will raise an OperationalError chained to
the OSError obtained from stat'ing the socket. Previously control would
have gone back to the generator with a read-ready state and it would
have failed on whatever libpq function would have touched the socket.

Fix the problem reported in #608 affecting the epoll wait function, for
which we opted to use the poll function instead (which, more simply,
made the closed fd easier to spot).

note that on Windows it's not possible to use os.fstat() to ckeck a
socket state, therefore make it no-op.