fix: prevent UNSAFE_TRANSACTION under concurrent sql.begin()

[dominikwagner]

Fixes the UNSAFE_TRANSACTION race reported in #823 (likely related to #827).

Root cause

When you call sql.begin(), the driver:

1. Picks a connection from the pool
2. Marks it reserved — "this connection belongs to this transaction, don't give it to anyone else"
3. Sends BEGIN to PostgreSQL
4. Runs your queries
5. Sends COMMIT and un-reserves the connection

Under concurrent load, the "reserved" mark can be lost or overwritten before step 4 finishes. Another sql.begin() grabs the same connection and sends its own BEGIN. PostgreSQL sees a second BEGIN while already in a transaction, warns 25001 "there is already a transaction in progress", and the driver throws UNSAFE_TRANSACTION.

Three code paths in src/connection.js and src/index.js can produce this:

1. execute() short-circuit — onexecute (which sets connection.reserved) is at the tail of an && chain. When sent.length >= max_pipeline (or write() returns false), the chain short-circuits and onexecute is skipped, so the connection is never marked reserved.
2. handler() pipelining — a queued BEGIN can be dispatched onto a busy connection that already holds another transaction.
3. drain() on reserved connection — the socket drain callback can call onopen() on a connection that is currently reserved, returning it to the pool mid-transaction.

Fix

Three small, focused changes:

Where	What it does
src/connection.js execute()	Fire onexecute unconditionally when write() succeeds, and return false so the connection is marked full (no further pipelining onto it).
src/connection.js drain()	Don't call onopen() on a reserved connection.
src/index.js handler()	Never dispatch an onexecute query (BEGIN) onto a busy connection — always queue it.

Reproduction

tests/race-condition.js reproduces the bug against unmodified PostgreSQL on localhost. It runs the scenario 20 times: pool max: 3, max_pipeline: 2, hold all 3 connections busy with pg_sleep, then fire 6 regular queries + 5 sql.begin() concurrently.

• On unfixed master: 12–16 / 20 iterations fail, PostgreSQL reports 25001 "there is already a transaction in progress"
• On this fix: 20 / 20 iterations pass

Setup:

createuser postgres_js_test
createdb -O postgres_js_test postgres_js_test
node tests/race-condition.js