Performance: don't validate ran changesets

If a changeset has already been run and should not run this time, in the validatingvisitor now does not validate the changeset.

Before CORE-1207 (https://github.com/liquibase/liquibase/pull/82) this was a 50% speedup. After this patch, because validation is much faster now, this appears to be about a 10% speedup.

I do not know if this causes trouble elsewhere - the tests run with the exact same result.

I added a few tests for this patch.