sqlite3: use one connection per transaction

[psarna]

This commit attempts to implement the following mechanism:

1. A single connection is created lazily whenever user executes a statement
2. When a transaction starts, the Transaction object takes ownership of the current transaction.
3. Subsequent queries will create a new connection.

That way we can safely run multiple transactions from a single client, without risking a "cannot begin transaction within transaction" error that comes from trying to send BEGIN twice on a single db connection.

[psarna]

@penberg NOTE: this is entirely untested yet, because I have no idea how to run tests for this typescript driver, and this is pretty much the first snippet of typescript code I ever produced. I'll keep researching how to validate this

[psarna]

@penberg any clue why CI with Vercel Edge fails? It doesn't look related to the patch