Removed 'restart' in Coro.

'restart' method of Coro is removed, as 'hotswap' can be used
for the same purpose ('hotswap' allows currently running
coroutine to inspect the new generator function for security or
to avoid mistakes, do any cleanup etc.).
Added 'close' method to Channel.

'close' method of Channel unregisters the channel and removes it
from channel list so it can't be used to subscribe or send messages.

'register' methods of Channel and RCI now don't take 'name' parameter;
the name used to initialize is used for registration (to avoid confusion).