Refactor Liveliness implementation

[fuzzypixelz]

Resolves https://github.com/eclipse-zenoh/zenoh/issues/864.

[eclipse-zenoh-bot]

@fuzzypixelz If this pull request contains a bugfix or a new feature, then please consider using Closes #ISSUE-NUMBER syntax to link it to an issue.

[eclipse-zenoh-bot]

@fuzzypixelz If this pull request contains a bugfix or a new feature, then please consider using Closes #ISSUE-NUMBER syntax to link it to an issue.

[eclipse-zenoh-bot]

@fuzzypixelz If this pull request contains a bugfix or a new feature, then please consider using Closes #ISSUE-NUMBER syntax to link it to an issue.

[fuzzypixelz]

What works now:

1. Liveliness declaration, w/o the interest protocol, in brokered scenarios
2. Liveliness get, w/o the interest protocol, in brokered scenarios

I disabled unused_variables and dead_code lints to unblock the CI.

[fuzzypixelz]

@OlivierHecart I've addressed your remarks in 303ba3f, 1ae0b4f, b27525d and faeb4a6. Next I'll be merging interests into the pull request branch. Then I'll finally move on implement queries using tokens.

[fuzzypixelz]

I just merged interests into the pull request branch. By the time I finished my first merge conflict resolution, there were more merge conflicts waiting for me. These development branches getting a bit unwieldy.

I marked interest_id fields as todo!() during the resolution. I'll fix that next.

[fuzzypixelz]

The interest_id fields in the liveliness implementation now correctly mirror the subscriber implementation.

[fuzzypixelz]

Update:

1. The .liveliness().get(..) API now sends interest declaration messages instead of requests.
2. Incoming token declaration messages are sent to liveliness query callbacks.