Add option to remove the default or null requirement when merges are in-order

[tantaman]

We only need the constraint that columns must have default values or be nullable if the merge order can be out of order.

For cases where users will be merging changes in-order we can do a two-phase commit against crsql_changes and relax this constraint.

The one extra caveat would be that syncs must be at transaction bounds. This could be problematic for long divergence given we lose some transaction information (see https://vlcn.io/blog/how-crsqlite-transactions-work-today)

So.. maybe back to a second virtual table for "full row" merges?

[tantaman]

Related to #294 (vtab for full row sync)