database: Start v3 module dev cycle.

[rstaudt2]

Changes have been made to the public API for the database module and, therefore, this follows the process for introducing major API breaks, which consists of:

• Bump the major version in the go.mod of the affected module if not already done since the last release tag
• Add a replacement to the go.mod in the main module if not already done since the last release tag
• Update all imports in the repo to use the new major version as necessary
• Make necessary modifications to allow all other modules to use the new version in the same commit
• Repeat the process for any other modules the require a new major as a result of consuming the new major(s)

The public API for the database module was changed in #2649, as a new Flush method was introduced.

[davecgh]

Thanks for the PR, but a major bump isn't needed for the addition of a new method. That is only a minor bump according to semver, so all that is needed to fix blockchain/v4 as of #2649 is to update the go.mod in blockchain to require the database/v2 module as of a commit at or after the addition of the new method.

[rstaudt2]

That makes sense, thanks for the clarification. In that case, I'll close this PR and do that instead.