[carbonlink] Python 3 Pickle Compatible Metric Request Parser

[alikhtag]

Problem

When graphite-web is installed on Python 3.0+ environments graphite-web will send pickle request to carbonlink using the highest pickle protocol (link to code in graphite-web) .

In python 3.0 until 3.7 highest pickle protocol is 4, while in python 3.8 highest is 5.

Since go-carbon's carbonlink was written for python 2.x, when highest protocol available was 2, it is unable to parse carbonlink pickle messages coming from python graphite-web. Even if the protocol is set to 2 when using python 3.x it won't be able to parse it due encoding change from ASCII in python 2.x to Unicode in python 3.x.

Steps to recreate
Run go-carbon normally, but run graphite-web in Python 3.0+ and request a metric that will utilize carbonlink. You will see protocol 4 message request coming *protocol 5 in Python 3.8+). Consequently go-carbon will WARN regarding bad pickle message error and wont serve metric from cache.

# go-carbon
WARN [carbonlink] request parse failed {"peer": "ip_address", "error": "Bad pickle message"} 
# Graphite web
ConnectionRefusedError: [Errno 111] Connection refused

PR Fixes

This PR should allow carbonlink to handle pickle protocols from 2 up to 5 by expanding the carbonlink pickle message parser to Python <= 3.8 pickle messages. More on python pickle protocols can be found here

Note: Will make PR later in graphite-web to make pickle protocol configurable for carbonlink requests, so that if newer python version with higher pickle protocol comes out it will not break carbonlink communication (one can set desired protocol) if go-carbon will not be able to parse it.

[deniszh]

Hi @azhiltsov @lomik @bom-d-van !
Could someone review and merge this PR, pretty please?
Much appreciated!

[alikhtag]

Thanks for merge @azhiltsov !