close() from a user function inside a write transaction's statement blocks #26

Found while reviewing project-minigraf/minigraf#824 (read from code, not reproduced).

MiniGrafDb.close() rolls back every open WriteTransaction by locking each transaction's slot (slot.lock()). While a transaction's statement runs, WriteTransaction::execute holds that slot. A user function that calls db.close() from inside that statement therefore blocks the JS thread on a mutex its own thread holds: the process hangs.

Expected: close() from inside a statement does not hang. Either it refuses with INT-001 (as a re-entrant call into the transaction already does via try_lock), or it closes the database and the running transaction is finished as soon as its statement returns, with later calls failing with API-018.

Possible fix: in close(), use try_lock on each slot. On WouldBlock (only possible through re-entry, since there is one JS thread), either refuse with INT-001, or mark the slot to be rolled back when execute releases it.

Refs project-minigraf/minigraf#824