WIP: Add the concept of incompatible schemas

[lithp]

What was wrong?

As part of https://github.com/ethereum/trinity/issues/779 py-evm will soon have to support reading and writing databases with a different schema, if the code ever tries to use a database of the wrong kind it might accidentally corrupt it. To prevent that from happening some checks are added which assert that the database is of the expected kind.

To-Do

• [ ] Clean up commit history

• [ ] Add entry to the release notes

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