auto calculate max_fee

[martriay]

instead of 0 as the default max_fee, nile should estimate the required fee for the users, which today have to guess it.

[ericnordelo]

I think it could make sense to wait for #320 before adding this (and I think it will be easier to add too).

320 adds an estimate_fee abstraction for all the transactions, that can be used for this (I think that addressing this without that could cause a lot of unnecessary conflicts in code and tests).

IMO it can still be part of the same milestone, as long as we merge #320 first.

[andrew-fleming]

Haha yeah, I stopped my progress after adding the implementation with account.send because of this. I'll checkout and work over the #320 PR. Thanks for the heads up, sir!