Remove ChannelManager class (dart-lang/json_rpc_2#64)

Fixes dart-lang/json_rpc_2#55

This class was intended to manage lifecycle around a `StreamChannel`,
but in the case of a `Peer` there is a conflict having 3 instances of a
`ChannelManager` attempting to manage lifecycle constraints which
results in completing the same `Completer` instance twice. Since `Peer`
needs a different behavior than `Server` or `Client` the
`ChannelManager` abstraction isn't helpful.

- Inline the important behavior around completing `done` appropriately
  for the end of the channel `stream`, or for errors, into `Peer`,
  `Server`, and `Client`.
- Inline the important behavior around closing the channel `sink` and
  completing `done` when a `Server` or `Client` is closed. `Peer` has
  different behavior and only forwards to it's `Server` and `Client`.
  The returned future from `channel.sink.close` is ignored, since this
  future does not complete in the case where a done even can't be
  delivered to listeners.