MAJOR: connection: remove the CO_FL_WAIT_{RD,WR} flags

MAJOR: connection: remove the CO_FL_WAIT_{RD,WR} flags

These flags were used to report the readiness of the file descriptor.
Now this readiness is directly checked at the file descriptor itself.
This removes the need for constantly synchronizing updates between the
file descriptor and the connection and ensures that all layers share
the same level of information.

For now, the readiness is updated in conn_{sock,data}_poll_* by directly
touching the file descriptor. This must move to the lower layers instead
so that these functions can disappear as well. In this state, the change
works but is incomplete. It's sensible enough to avoid making it more
complex.

Now the sock/data updates become much simpler because they just have to
enable/disable access to a file descriptor and not to care anymore about
its readiness.