Implement full MVCC support

[ecton]

There are two patterns of MVCC we want to make sure to support:

• The ability to make multiple read queries from a database with the guarantee they're all done on the same transaction.
• The ability to track read operations during a transaction and verify nothing read has changed when the transaction executes.

This issue needs functionality from Nebari:

• Blocked by khonsulabs/nebari#32

It might be better to have the read-operation verification happen inside of Nebari as well.