Until now, we've used a custom comparer in Pebble to achieve the
hiearchical sorting.

This PR is changing it to use a key-encoding scheme and byte-wise
sorting instead. There are a few reasons:

1. Custom comparer is not working anymore in Pebble 2.1. There are
several additional restrictions that makes it possible to continue using
the current compare function. We are locked out of upgrading Pebble if
we don't change this.
2. We need to upgrade to Pebble 2 because there is a conflict version
with ZStd depedency. Newer Zstd is making Pebble 1.x crashing. There are
other dependencies that start requiring new Zstd, such as the
hashicorp/raft dep in #757.
3. All in all, this will make all the internal DB operations faster,
since after the keys are encoded in the wrapper layer, than all the
comparisons will just by byte-wise comparison (eg: db iterators,
compactions, ...)

There is a new marker added by Oxia to the db so we know the key
encoding format. Later, we will add more encoding option (eg: natural
sorting).

The first time a db is opened, if the marker is not set, it will be
converted to new format.

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Signed-off-by: Matteo Merli <mmerli@apache.org>