Added flag to databaseChangeLog to allow skipping of checksum validation.

Currently, there's no way to make improvements to old changesets without breaking users who have already ran those change sets. This approach allows the ability to skip validation for a change log.

This is a rework of my first attempt at this.
My first approach was too coarse grained, it removed the validation of the actual
changes in a changeset. This approach just prevents the validation of checksums for
changesets that have already ran.

Added integration test for the new functionality.
Tested using H2IntegrationTests.