A second connection made from the first

[tamnd]

Milestone tamnd/zu#169, item 6: cursor() in both clients, which is how a pool is written. The engine half is tamnd/zu#409 and the Python half is tamnd/zu-python#28, and this pins the engine.

conn.duplicate() gives back another Connection on the same database. The name is not cursor() here, which is what the other clients call it: conn.cursor() in this client is already a cursor over the rows of one statement, and a different thing entirely, so this one says what it does. The README says so where a reader coming from Python will look.

It forks off the database the connection already holds rather than opening the file again, so it costs a schema load and no path lookup, and it works on a database in memory, where there is no path to open a second time. That was the gap worth closing: a pool that seeds itself and lets the first connection go had no way to a second one at all.

The two are connections in every sense rather than two names for one. Each has its own prepared statements, its own caches and its own transaction, so a task taking one from a pool is not in whatever transaction the last borrower left open, and closing one does not close the other. What they share is the write side, so they queue behind each other to write and each sees what the other has committed. Two of them read at once, where two statements on one connection queue, and there is a test that runs a pair through Promise.all.

It is a task like every other call that reaches the engine, because a schema load on the runtime's thread is the loop stopped for the length of one. The switches come across, including bigIntMode and temporal, and there is a test that a duplicate of a bigIntMode: 'number' connection still answers numbers.

Eight tests: the pool case both ways, the properties carried across, a duplicate outliving the connection it was made from, a transaction of its own, the spelling carried across, two reading at once, a closed connection refused by name, and await using on a duplicate. Plus the README program.

npm test, npm run check:types, npm run check:api, cargo clippy -- -D warnings and cargo fmt --check all green locally.