Correct mDNS TCP/UDP socket flags

* Explicitly mark TCP and UDP as blocking sockets
* Added socket.SO_REUSEADDR to UDP socket
* Added socket.SO_REUSEPORT to TCP+UDP sockets
* Moved socket creation to inside the worker thread's
  start() method.
* Added a socket.close() to the stop() method.
* Enable eventlet patching of the "os" module, required
  to avoid symptoms which look near identical to [1].

[1]: https://github.com/eventlet/eventlet/issues/210

Closes-Bug: 1444384
Change-Id: Iff642673a33d7b8d2e2c1af833f944330001c33b

designate/dnsutils.py

@@ -247,7 +247,9 @@ def bind_tcp(host, port, tcp_backlog):
              {'host': host, 'port': port})
     sock_tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     sock_tcp.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    sock_tcp.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
     sock_tcp.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
+    sock_tcp.setblocking(True)
     sock_tcp.bind((host, port))
     sock_tcp.listen(tcp_backlog)
 
@@ -259,6 +261,9 @@ def bind_udp(host, port):
     LOG.info(_LI('Opening UDP Listening Socket on %(host)s:%(port)d') %
              {'host': host, 'port': port})
     sock_udp = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock_udp.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    sock_udp.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
+    sock_udp.setblocking(True)
     sock_udp.bind((host, port))
 
     return sock_udp