core: supports incremental sync

[maxux]

This pull request is an early adoption of a new way to synchronize database incrementally. Some features:

• Ensure checksum of index/data files doesn't change across zdb restart
  • Restarting zdb was updating a timestamp on database headers, changing checksum after restart even if data didn't changed.
• Add new commands to know and keep track of current database position
• Add commands to fetch raw entry data