Refactor pt-table-checksum Internals

Several years ago, Baron and I talked about refactoring pt-table-checksum's internals. The checksum and other algorithms are good, but we need to improve the overall design and implementation of the tool in order to make it more reliable, easier to test, and easier to implement or change features. This will be critical if we implement https://blueprints.launchpad.net/percona-toolkit/+spec/pt-table-checkum-only-replicate