feat(sdk): Allow LinkedChunkUpdatesInner to have multiple readers.

This patch removes the notion of `take` vs. `peek` from
`LinkedChunkUpdatesInner` and widens the problem to a more general
approach: `LinkedChunkUpdatesInner` must support multiple readers, not
only two (`take` was the first reader, `peek` was the second reader,
kind of).

Why do we need multiple readers? `LinkedChunkUpdates::take` is
clearly the first reader, it's part of the public API. But the private
API needs to read the updates too, without consuming them, like
`LinkedChunkUpdatesSubscriber`. `peek` was nice for that, but it's
possible to have multiple `LinkedChunkUpdatesSubscriber` at the same
time! Hence the need to widen the approach from 2 readers to many
readers.

This patch introduces a `ReaderToken` to identify readers. The last
indexes are now all stored in a `HashMap<ReaderToken, usize>`. The rest
of the modifications are the consequence of that.

The test `test_updates_take_and_peek` has been entirely rewritten to be
`test_updates_take_and_garbage_collector` where it tests 2 readers and
see how the garbage collector reacts to that.

Author: Hywan