First message on an interface is server.version (#4407)

- Use server.ping to ping (valid as of protocol version 1.2) rather
  than server.version (deprecated as a ping)
- Ping every 5 mins rather than every 1 minute.  By default servers
  don't consider a connection stale until 10 minutes of inactivity.
- Remove unused last_request member of interface

Port of c3f26e59db

lib/network.py

@@ -307,9 +307,6 @@ class Network(util.DaemonThread):
         # Resend unanswered requests
         requests = self.unanswered_requests.values()
         self.unanswered_requests = {}
-        if self.interface.ping_required():
-            params = [ELECTRUM_VERSION, PROTOCOL_VERSION]
-            self.queue_request('server.version', params, self.interface)
         for request in requests:
             message_id = self.queue_request(request[0], request[1])
             self.unanswered_requests[message_id] = request
@@ -711,6 +708,9 @@ class Network(util.DaemonThread):
         interface.mode = 'default'
         interface.request = None
         self.interfaces[server] = interface
+        # server.version should be the first message
+        params = [ELECTRUM_VERSION, PROTOCOL_VERSION]
+        self.queue_request('server.version', params, interface)
         self.queue_request('blockchain.headers.subscribe', [True], interface)
         if server == self.default_server:
             self.switch_to_interface(server)
@@ -734,8 +734,7 @@ class Network(util.DaemonThread):
             if interface.has_timed_out():
                 self.connection_down(interface.server)
             elif interface.ping_required():
-                params = [ELECTRUM_VERSION, PROTOCOL_VERSION]
-                self.queue_request('server.version', params, interface)
+                self.queue_request('server.ping', [], interface)
 
         now = time.time()
         # nodes