Sled serde

[alexsnaps]

This PR is broken down in a few steps:

• Changed the key functions to work with bytes, rather than String - d5455da
  Still allocates on the heap, but avoids validating the UTF-8 sequences
• Swaps out the JSON serialization for the binary Postcard one - ab3ed00
• Fixes Infinispan by base64 encoding the binary keys - 3db3c09
• Finally, reverted to String based JSON encoding for all storages other than the disk one - ca03986
  • This keeps backwards compatibility and avoids that dance with the Base64 encoding
  • On the flip side, the keys and counters occupy more memory in the backend service

With changing to [postcard](https://postcard.jamesmunns.com/), we get a 8% increase this scenario, which is the worst improvement. Others go up to ~15% improvement. A key is about 3x smaller in size.

[eguzki]

LGTM

[alexsnaps]

Closed in favor of #178