Fix connection reaper deadlock.

Added explicitly running test to detect deadlock.
Use ```go test -tags=TestConnReaperDeadlockLoop -run ^TestConnReaperDeadlockLoop$```.
Be aware the test generate a lot of sockets in TIME_WAIT.
Without patch to socket.go it breaks pretty quick on fast and slow machines.
With the patch it runs way longer but gets slower when there are plenty of sockets in TIME_WAIT.

socket.go

@@ -11,6 +11,7 @@ import (
 	"log"
 	"net"
 	"os"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -384,10 +385,16 @@ func (sck *socket) connReaper() {
 			return
 		}
 
-		for _, c := range sck.closedConns {
+		// Clone the known closed connections to avoid data race
+		// and remove those under reaper unlocked.
+		// That would be resoling deadlock from #149 simpler way.
+		cc := slices.Clone(sck.closedConns)
+		sck.closedConns = nil
+		sck.reaperCond.L.Unlock()
+		for _, c := range cc {
 			sck.rmConn(c)
 		}
-		sck.closedConns = nil
+		sck.reaperCond.L.Lock()
 	}
 }
 

socket_loop_test.go

@@ -0,0 +1,27 @@
+//go:build TestConnReaperDeadlockLoop
+
+package zmq4_test
+
+import (
+	"log"
+	"testing"
+	"time"
+)
+
+// Use ```go test -tags=TestConnReaperDeadlockLoop -run ^TestConnReaperDeadlockLoop$```
+func TestConnReaperDeadlockLoop(t *testing.T) {
+	n := uint64(0)
+	for {
+		n++
+		if n%100 == 0 {
+			log.Printf("%d ...", n)
+		}
+		timer := time.AfterFunc(30*time.Second, func() {
+			log.Fatalf("failed at %d!!!", n)
+		})
+
+		TestConnReaperDeadlock(t)
+
+		timer.Stop()
+	}
+}