Rewrite channels to expose a sender interface

[msimberg]

Currently the various channel implementations in pika expose a low level API, similar to lockfree queues, where one can get an item through bool channel(T*). A more interesting API would return senders to signal that an item is available from the channel.

For transitioning we could:

• Remove the current implementations and add a senderized version separately (#1209)
• Keep the current implenmentations until we have a senderized version
• Keep the non-sender version indefinitely

[msimberg]

Related: #989.

[msimberg]

I'm leaving this open to track the possibility of a adding a senderified channel implementation (despite removing the current non-sender implementation in #1209).