support pickle protocol versions 0, 1, 2, 3 & 4 + accept pickle arrays + send pickle tuples

[DanCech]

Fixes #340

We have this check in place to drop obviosuly-invalid data sent to the pickle port, rather than trying to feed it into og-rek.

With this update we should cover all possible valid payload prefixes for pickle protocol versions 0 through 4.

To verify, you can send a test metric with a command like:

python -c 'import pickle, struct, os; payload = pickle.dumps([("metric",(1,2))],protocol=2); os.write(1, struct.pack("!L", len(payload)) + payload)' | nc localhost 2014

Replace protocol=2 to test other supported versions, to use protocol 3 or 4 you need to be running python 3.x

[Dieterbe]

I can confirmed that when doing carbon-relay-ng -> carbon-relay-ng using pickle, the following error

pickle.go: Unrecognized type []interface {} for item.

... is fixed

1. by the 2nd commit
2. if you isolate out only the receiver side fix from the 2nd commit
3. if you isolate out only the sender side fix from the 2nd commit

so we were sending out []interface{} (which becomes a pickled array/list), which is the same as what some other tools do,
but we were only accepting pickled tuples (like carbon-relay.py sends, but not our own code itself)
inspired by buckytools (
https://github.com/jjneely/buckytools/blob/f3f6c69e2987035d3c0dd3fcbc24bf3afb2e9023/cmd/bucky-pickle-relay/main.go#L254) we are now more permissive on the receiver side, as well as send the data more cleanly as a tuple just like python code.

[yanlibert]

This is exactly what I needed. I had some problem upgrading to the latest version in production due to some pickle data being dropped. It's all fine now. Thanks a lot for the good work!