Read-write transactions with unbounded size

[losfair]

Currently there is a 50,000 pages (390 MiB) total write size limit on transactions, but larger transactions can be useful in scenarios like schema change.

It is possible to provide an option to trade write concurrency for unbounded read-write transactions by implementing commit locks. I did some work on this in https://github.com/losfair/mvsqlite/pull/49, but closed the PR due to the complexity - maybe we should re-consider in the future.