When a server shuts down, the fiber channel is never closed

[jwoertink]

During the Server#shutdown, this fiber_channel never calls close. I'm not sure if Crystal closes them automatically at some point, or if that even matters, but we should add that in https://crystal-lang.org/api/1.20.2/Channel.html#close%3ABool-instance-method

cable/src/cable/server.cr

Line 25 in bfdb55d

getter fiber_channel = ::Channel({String, String}).new

It should go after all this

cable/src/cable/server.cr

Lines 193 to 196 in bfdb55d

	@connections.values.dup
	end
	connections_to_close.each do |connection|
	connection.close