Change min fee from constant to suggested param

[plasmatech8]

I do not know what peoples thoughts are about this is since I did not receive a response from #284.

This PR changes the fee calculations for Transaction classes under future.transaction so that it uses sp.min_fee instead of constants.min_txn_fee in calculations when flat_fee=false.

Rationale:

• It is more intuitive to use min_fee in the SuggestedParams which the user is actively passing into the Transaction object, rather than a hidden/hard-coded value.
• Application calls with inner transactions require a higher min-transaction fee. It is more convenient to set the value suggested_params.min_fee *= 2 rather than configuring flat fees.
• The min-fee might change in the future, and it is better to use the value returned from the Algorand REST API than a hard-coded constant.

I tried running tests using make docker-test but it is excruciatingly slow for some reason, so I will think about that another time.

[CLAassistant]

All committers have signed the CLA.

[algochoi]

Thanks for your patience - I think the SuggestedParameters may or may not have the min_fee field, and it's causing our current tests to fail:

 File "/py-algorand-sdk/algosdk/future/transaction.py", line 369, in __init__
    self.estimate_size() * self.fee, sp.min_fee
TypeError: '>' not supported between instances of 'NoneType' and 'int'

[jannotti]

I agree, we should not use constants.min_txn_fee when we have better information available. Ideally, we'd some day be able to remove the constant entirely. If you want to rework this (there's currently a conflict), I help see through to merge. If not, I'll try to get the same change in soon.

[jannotti]

It'll go in in #530