Registers this channel with the given selector, returning a selection key.
This method first verifies that this channel is open and that the given initial interest set is valid.
If this channel is already registered with the given selector then the selection key representing that registration is returned after setting its interest set to the given value.
Otherwise this channel has not yet been registered with the given selector, so the register
method of the selector is invoked while holding the appropriate locks. The resulting key is added to this channel's key set before being returned.