Run queries on a connection one at a time so transactions stay on one session

[just-some-random-pal]

Fixes #191.

Every request carries the session the previous response returned, and Connection.execute only stores the new session after the response arrives. Two requests in flight on one connection both send the stale session, and the server treats the second as a new session or rejects it with "transaction ... in use". Inside transaction() that means a query runs outside the transaction, or COMMIT/ROLLBACK runs on a session with no transaction while the real one stays open until it times out, blocking every writer on the table. The failure path in #191 is the common one: the callback throws while a query it started is still running, ROLLBACK is sent immediately with the pre-query session, and the update's transaction is never rolled back.

This serializes requests per Connection through a FIFO chain, the same approach mysql2 takes per connection and the one discussed on #139. Each request now runs after the previous one has settled, so it always carries that response's session. A failed request does not block the ones behind it, so ROLLBACK still runs after a failed query. COMMIT and ROLLBACK therefore wait for every query the callback started, awaited or not, and run on the session that query produced. refresh() goes through the same chain.

What changes for callers: queries on one Connection no longer overlap. Client.execute and Client.transaction already create a connection per call, and Client.connection() hands out new ones, so parallel work across connections is unaffected. Promise.all over one connection still works, it just runs the queries in order, which is the only order the session protocol can honor. The README's transaction section now says so.

Tests, with a fetch that returns a new session per response and can hold a response open: a second query is not sent while the first is in flight and carries its session; a failed query does not block the next; a callback that throws while its query is running sends ROLLBACK only after that query completes and with its session; un-awaited inserts run before COMMIT. All four fail on main and pass with the change. The full suite is 46/46 and npm run lint is clean.