Add a __str__ method to the ClientHandler class

So the daemon stop reports:

n1.osvcd.listener stop client handler thread (client addr: 192.168.99.16, usr: lva, auth: x509, scheme: h2)
n1.osvcd.listener stop client handler thread (client addr: local, usr: False, auth: uxsock, scheme: h2)
n1.osvcd.listener stop client handler thread (client addr: local, usr: False, auth: uxsock, scheme: h2)
...

Instead of:

n1.osvcd.listener stop <ClientHandler(Thread-26, started 139992432494336)>
n1.osvcd.listener stop <ClientHandler(Thread-50, started 139992412727040)>
n1.osvcd.listener stop <ClientHandler(Thread-1068, started 139992424101632)>
...

lib/osvcd_lsnr.py

@@ -584,6 +584,14 @@ def __init__(self, parent, conn, addr, encrypted, scheme, tls, tls_context):
         self.usr_grants = {}
         self.events_counter = 0
 
+    def __str__(self):
+        return "client handler thread (client addr: %s, usr: %s, auth: %s, scheme: %s)" % (
+            self.addr[0],
+            self.usr.svcname if self.usr else self.usr,
+            self.usr_auth,
+            self.scheme
+        )
+
     def run(self):
         try:
             close = True