Fee Pooling SDK work (Go & Java)

[jannotti]

Once #2173 is merged, SDKS will want to support creating transactions with a fee under mintxnfee, because other txns in the group will be paying. The python sdk, and probably others, automatically increase the fee on your txn to the minimum, and/or they treat 0 as meaning "please fill in the suggested fee". We'll need an interface that allows setting these low fees without the SDK "fixing" them.

Notes:

• If using flat fee option and set it to 0, then accept.
• Confirm the encoded transaction does not contain a 0 fee field if the field is 0 (should just be missing)

[winder]

Yes, they all set the fee to min(provided, 1000).

I believe all SDKs have a "FlatFee" option, maybe we can change its behavior to ignore the minimum value