Add option for human readable key encoding

[kloboves]

Currently keys are encoded using a binary encoding scheme which makes it very hard to inspect the state of the database. It makes it hard to search for currently stored updates for a certain document (hard to construct the right prefix - for example if using RocksDB with ldb scan tool).

Since the keys are very simple (doc name + clock ID), I think it would be better to give an option to store them in a simple string format (something like :<clock_id> or whatever separator).

This could be easily implemented by exposing the key encoder / decoder as part of the API.

[dmonad]

Sure, feel free to open a PR 👍