feat(sdk) Add the LinkedChunkUpdates::peek.

This patch adds the `LinkedChunkUpdates::peek` method. This is
a new channel to read the updates without consuming them, like
`LinkedChunkUpdates::take` does.

The complexity is: when do we clear/drop the updates then? We don't
want to keep them in memory forever. Initially `take` was clearing
the updates, but now that we can read them with `peek` too, who's
responsible to clear them? Enter `garbage_collect`. First off,
we already need to maintain 2 index, resp. `last_taken_index` and
`last_peeked_index` so that `take` and `peek` don't return already
returned updates. They respectively know the index of the last update
that has been read. We can use this information to know which updates
must be garbage collected: that's all updates below the two index.
Tadaa. Simple. The only _drawback_ (if it can be considered as such)
is that the garbage collection happens on the next call to `take` or
`peek` (because of the borrow checker). That's not really a big deal in
practise. We could make it happens immediately when calling `take` or
`peek` but it needs more pointer arithmetic and a less straighforward
code.