fix(server): close conns channel only after accept loops exit

[m-u-xyz]

Problem

Listener.Close raced with its own accept loops, causing intermittent
send on closed channel panics and -race failures.

The accept loops send accepted connections into l.conns via a select
that also watches l.shutdown:

select {
case <-l.shutdown:
    conn.Close()
    return nil
case l.conns <- connRes{conn, err}:
}

Go's select picks randomly among ready cases. Close did:

l.once.Do(func() {
    close(l.shutdown)
    close(l.conns)   // <-- while a loop may still be mid-send
})
return l.eg.Wait()

So a loop that had just accepted a connection could win the conns <-
arm and send into a channel Close was closing concurrently →
runtime.closechan vs runtime.chansend race (panics or trips the
detector). This showed up downstream in dolt's sql-server tests as a
flaky race on server/listener.go:101 vs :149 during rapid
start/stop.

Fix

Split the single sync.Once into shutdownOnce and connsOnce so
Close can sequence the two channel closings around eg.Wait():

1. close(shutdown) — loops observe it and return
2. close the underlying net.Listeners — unblocks Accept(), loops exit
3. eg.Wait() — all senders are now guaranteed gone
4. close(conns) — safe; no sender remains

Accept()'s contract is preserved: it still unblocks with
net.ErrClosed because conns is still closed, just after the loops
drain rather than before.

Reproduction / test

server/listener_race_test.go adds a deterministic repro that:

• starts a Listener but withholds the Accept() consumer so the
  loop's conns <- send stalls inside the select,
• dials several real connections so the loop has a conn to send,
• calls Close while that send is in flight,
• repeats 50×.

On the original code this fails under -race:

WARNING: DATA RACE
--- FAIL: TestListenerCloseNoRace
runtime.closechan  (server/listener.go:149)
runtime.chansend   (server/listener.go:101)

On the patched code it passes (-race -count=5 clean). Covers both the
TCP and unix accept loops (they share conns) plus the post-Close
Accept → net.ErrClosed contract.

Checklist

• go build ./...
• go vet ./server/
• go test -race -count=5 -run TestListenerClose ./server/
• Existing server package tests still pass
• Test fails on the unfixed code (proven via stash/revert)