Using calculate_fee() to set fee may result in incorrect fee amount

[theoreticalbts]

Fee size is based on operation size, which may change when the fee is set (depending on how the fee is serialized, in particular, whether either the share_type or asset_id_type is serialized as a variable length integer).

In other words, after set_fee assigns the fee to the fee field, the result is (in some sense) a different operation which might have a different fee (i.e., calling calculate_fee() on the newly produced operation may have a different result from calling calculate_fee() on the original, zero-fee operation).

Currently this is completely ignored, resulting in the possibility that some transactions might have set_fee calculate a slightly incorrect fee -- resulting in breaking its contract of computing the minimal fee that the chain will accept. If the fee is too high it is not bad, but in the main real-world case, the fee will be being set from zero, which is much more likely to result in an expansion of the operation's serialized form than a contraction. Net result: an operation that will not validate due to insufficient fees even after setting the fee to the result of calculate_fee(), calling set_fee() on your operation, or using the get_required_fees() API call.

[theoreticalbts]

Merged to develop, awaiting review.

[theoreticalbts]

Merged.