Add optional C extension wrapper for Python JSON parsing

The pure Python in-tree JSON parser is *much* slower than the
in-tree C JSON parser. A local test parsing a 100Mb JSON file
showed the Python version taking 270 seconds. With the C wrapper,
it took under 4 seconds.

The C extension will be used automatically if it can be built. If
the extension fails to build, a warning is displayed and the build
is restarted without the extension.

The Serializer class is replaced with Python's built-in
JSON library since the ability to process chunked data is not
needed in that case.

The extension should work with both Python 2.7 and Python 3.3+.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

python/ovs/json.py

@@ -18,6 +18,11 @@ import sys
 import six
 from six.moves import range
 
+try:
+    import ovs._json
+except ImportError:
+    pass
+
 __pychecker__ = 'no-stringiter'
 
 escapes = {ord('"'): u"\\\"",
@@ -165,6 +170,12 @@ class Parser(object):
     # Maximum height of parsing stack. #
     MAX_HEIGHT = 1000
 
+    def __new__(cls, *args, **kwargs):
+        try:
+            return ovs._json.Parser(*args, **kwargs)
+        except NameError:
+            return super(Parser, cls).__new__(cls)
+
     def __init__(self, check_trailer=False):
         self.check_trailer = check_trailer