Various fixes and neats for the metered-channel

[vstakhov]

Here is the list of tasks:

• [x] Use async_channel for unbounded channels
• [x] Add unit tests for the unbounded channels
• [x] Update versions and dependencies - orchestra has added some incompatible changes over the errors handling, hence it is good to follow semver here
• [x] Use thiserror in the example to ensure we are compatible with the ordinary use case
• [x] Added len methods for all senders and receivers, because we have this information now