msgpack format

[DanCech]

graphite-project/graphite-web#2123 adds support for using msgpack as a transport encoding for intra-cluster find and render requests which is substantially faster than pickle for large results.

It uses the same data structure as the pickle format, except that intervals are encoded as 2-element arrays rather than interval objects (which is actually how MT encodes them in its pickle output anyway), and the mime type application/x-msgpack

We should add support for format=msgpack so we can take advantage of the better performance. For the most part this should just be a matter of using a msgpack encoder instead of pickle and sending the right content-type header, though we will also need to tweak the NaN handling to make sure they are properly encoded as nil values in the msgpack output.

[Dieterbe]

my only remark here is that msgp (currently existing format) vs msgpack is not clear.
would it make sense to call it msgp-graphite instead? (minor point, i don't feel strongly about this. we could also just document it)

[DanCech]

no, graphite sends format=msgpack

[Dieterbe]

fixed by #789