database: Introduce v2 module.

[davecgh]

In order to make use of dcrutil/v2, a new major version of the database module is required because database/v1 accepts and returns types from dcrutil/v1 in its API.

Since a new major version is required, this also takes the opportunity break the tight coupling with dcrutil by introducing a new BlockSerializer interface and accepting it instead of *dcrutil.Block in the database interface API.

Consequently, this introduces database/v2 which addresses the aforementioned items. A series of individual commits is provided to make the review process easier. Each commit message more thoroughly describes its purpose, but primarily they consist of the following:

• Freeze version 1 of the database module
• Remove the root module database override so building the software will still produce binaries based on the v1 module until the v2 module is fully released
• Introduce a new BlockSerializer interface that is limited specifically to ability to obtain the hash and serialization of a block and update the StoreBlock method that previously accepted *dcrutil.Block to use the new interface instead
• Update to use dcrutil/v2@v2.0.0
• Bump the module version to v2
• Update all examples, tests, internal imports, and the dbtool utility to import database/v2
• Update docs for the new module version

Finally, it should also be noted that this only introduces the new module and does not update anything to make use of it yet, so building the software will still produce binaries based on the v1 module.