Add kill flag to AprsClient

When starting an AprsClient with AprsClient.run(...) the client enters
a loop without an exit condition (i.e. a while True loop).  If autoreconnect
is set to True, it is impossible to exit the aforementioned loop even if
AprsClient.disconnect() is called.

This causes problems when running the client in a thread (or as a
background service, etc.) as the process will not join nor terminate
unless explicitly shutdown with SIGKILL.

Minimal example:

```
import signal

from ogn.client import AprsClient
from ogn.parser import parse_aprs, parse_ogn_beacon, ParseError

def process_beacon(raw_message):
    print('Received message')

client = AprsClient(aprs_user='N0CALL')
signal.signal(signal.SIGTERM, lambda signo, stackno: client.disconnect())
client.connect()
client.run(callback=process_beacon, autoreconnect=True)
```

This commit fixes such issues by adding a kill flag that is raised when
calling AprsClien.disconnect() to the while conditions of both loops inside
AprsClient.run().

Note: the outermost loop could still remain a while True loop as the
exit condition is checked at the end of the loop body.

ogn/client/client.py

@@ -19,6 +19,8 @@ class AprsClient:
         self.aprs_user = aprs_user
         self.aprs_filter = aprs_filter
 
+        self._kill = False
+
     def connect(self):
         # create socket, connect to server, login and make a file object associated with the socket
         self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -45,11 +47,13 @@ class AprsClient:
         except OSError:
             self.logger.error('Socket close error', exc_info=True)
 
+        self._kill = True
+
     def run(self, callback, timed_callback=lambda client: None, autoreconnect=False):
-        while True:
+        while not self._kill:
             try:
                 keepalive_time = time()
-                while True:
+                while not self._kill:
                     if time() - keepalive_time > settings.APRS_KEEPALIVE_TIME:
                         self.logger.info('Send keepalive')
                         self.sock.send('#keepalive\n'.encode())
@@ -71,7 +75,7 @@ class AprsClient:
             except socket.error:
                 self.logger.error('socket.error', exc_info=True)
 
-            if autoreconnect:
+            if autoreconnect and not self._kill:
                 self.connect()
             else:
                 return

tests/client/test_AprsClient.py

@@ -42,6 +42,7 @@ class OgnClientTest(unittest.TestCase):
         client.disconnect()
         client.sock.shutdown.assert_called_once_with(0)
         client.sock.close.assert_called_once_with()
+        self.assertTrue(client._kill)
 
     def test_50_live_messages(self):
         print("Enter")