Add opt-in reject_throttle: pace connection opens after establishment failures

[fabio-vapi]

Fixes #1179.

Adds an opt-in reject_throttle (default off; when off, behaviour is identical to today) that fixes the establishment-failure herd described in the issue.

What it does

• Backoff on establishment. Establishment retries now use the existing backoff curve instead of retrying immediately. (Today closed()'s if (initial) return reconnect() returns before closedTime/delay are assigned, so reconnect() computes setTimeout(connect, 0) — a tight loop.)
• Coordinate the pool. On an establishment failure, one failed connection becomes the lone prober (retrying with backoff) while every other connection's open is blocked. On each prober handshake-success an exponentially-growing batch of blocked connections is admitted (slow-start, 1 → 2 → 4 → … → max); a failed admission collapses the ramp back to 1. Once the backlog drains, the breaker disengages back to baseline. Liveness is the prober's own retry — no timers.

It complements the existing backoff rather than duplicating it: backoff only delays each retry, and because its counter lives on the shared pool object, every connection waits the same delay and then fires together — a delayed herd. reject_throttle keeps exactly one attempt in flight during an outage and ramps back up on recovery.

It governs only the clean-close establishment-reconnect path; error-close (RST) and connect-timeout still reject exactly as before, so it never masks a hard failure. With it enabled, the issue's reproduction drops from ~12.5k attempts/sec to single digits and recovers via slow-start once the endpoint accepts again.

Changes

• src/index.js — the reject_throttle option and shared.throttle state; the gate (throttleAllows) at the three closed → connecting promotion sites; throttleAdmit (slow-start admission + disengage) in onopen; requeue.
• src/connection.js — establishment-failure routing (reconnect_with_reject_throttle → prober_reconnect | stand_down), backOffOnError, and throttle_on_success at ReadyForQuery.

Tests

tests/reject_throttle.js — 21 cases: option plumbing, throttle-vs-herd, single prober, slow-start recovery and disengagement (including large backlogs and max: 1), the drop path staying intact via max_lifetime, and the boundaries (RST reject, connect_timeout, off == vanilla). Wired into test:esm; kept as a separate file for reviewability — happy to fold into tests/index.js if you'd prefer.