Improve UDPTransport.sendto address validation (fixes: #214)

When UDPTransport.sendto is called with a destination address, only
perform the validation once and store the result in an LRU cache. Also
store some of the socket's properties (family, proto, type) as integers
to avoid repeatedly fetching these properties and converting them to integers.

uvloop/handles/udp.pxd

@@ -1,6 +1,9 @@
 cdef class UDPTransport(UVBaseTransport):
     cdef:
         object sock
+        int sock_family
+        int sock_proto
+        int sock_type
         UVPoll poll
         object address
         object buffer

uvloop/handles/udp.pyx

@@ -1,4 +1,5 @@
-import socket
+import functools
+
 
 cdef class UDPTransport(UVBaseTransport):
 
@@ -17,6 +18,9 @@ cdef class UDPTransport(UVBaseTransport):
             socket_inc_io_ref(sock)
 
             self.sock = sock
+            self.sock_family = sock.family
+            self.sock_proto = sock.proto
+            self.sock_type = sock.type
             self.address = r_addr
             self.poll = UVPoll.new(loop, sock.fileno())
             self._finish_init()
@@ -143,18 +147,8 @@ cdef class UDPTransport(UVBaseTransport):
             raise ValueError(
                 'Invalid address: must be None or {}'.format(self.address))
 
-        if addr is not None and self.sock.family != socket.AF_UNIX:
-            addrinfo = __static_getaddrinfo_pyaddr(
-                addr[0], addr[1],
-                uv.AF_UNSPEC, self.sock.type, self.sock.proto, 0)
-            if addrinfo is None:
-                raise ValueError(
-                    'UDP.sendto(): address {!r} requires a DNS lookup'.format(
-                        addr))
-            if addrinfo[0] != self.sock.family:
-                raise ValueError(
-                    'UDP.sendto(): {!r} socket family mismatch'.format(
-                        addr))
+        if addr is not None and self.sock_family != uv.AF_UNIX:
+            self._validate_address(addr)
 
         if self._conn_lost and self._address:
             if self._conn_lost >= LOG_THRESHOLD_FOR_CONNLOST_WRITES:
@@ -188,3 +182,17 @@ cdef class UDPTransport(UVBaseTransport):
         # Ensure that what we buffer is immutable.
         self.buffer.append((bytes(data), addr))
         self._maybe_pause_protocol()
+
+    @functools.lru_cache()
+    def _validate_address(self, object addr):
+        addrinfo = __static_getaddrinfo_pyaddr(
+            addr[0], addr[1],
+            uv.AF_UNSPEC, self.sock_type, self.sock_proto, 0)
+        if addrinfo is None:
+            raise ValueError(
+                'UDP.sendto(): address {!r} requires a DNS lookup'.format(
+                    addr))
+        if addrinfo[0] != self.sock_family:
+            raise ValueError(
+                'UDP.sendto(): {!r} socket family mismatch'.format(
+                    addr))