Cache imported protocol codecs

There was no measurable difference in test runtime with this, but `timeit`
reports a factor 10 speed-up when calling `find_codec_by_name` in a tight
loop, so it may make a difference in some cases.

Author: ntninja

multiaddr/codec.py

@@ -8,10 +8,14 @@
 from .protocols import read_varint_code
 
 
+CODEC_CACHE = {}
 def find_codec_by_name(name):
     if not name:
         raise ValueError("unknown protocol codec")
-    return importlib.import_module(".codecs.{0}".format(name), __package__)
+    codec = CODEC_CACHE.get(name)
+    if not codec:
+        codec = CODEC_CACHE[name] = importlib.import_module(".codecs.{0}".format(name), __package__)
+    return codec
 
 
 def string_to_bytes(string):