2 - support multi column primary keys
3 - rename Change to ChangeLog to avoid conflict with mysql reserved word
4 - fix schema composition when sharing connection
5 - support for nested transactions (produce one changeset, but support stubs
6 for nested changesets in place)
8 - various other cleanups
9 - calling current_changeset outside of txn_do is fatal
10 - fix 'now()' default value, timestamp happens in perl space instead
11 - also call txn_do on the journal schema if the storage object differs
12 - don't call txn_begin twice