Conside removing indexes from `envelpeData` structure

[AnnaShaleva]

Rationale

Described in #287 (comment) and #287 (comment).

Implementation

If we agree to prioritize Envelopes over simple transactions and always put them in the start of the transactions list then index field should be removed from envelopeData structure.

Depends on #286.

[txhsl]

#286 is still a reasonable solution to prioritize Envelope transactions, although we closed it. But it seems good to keep the original transaction order, at least for now, so this is not urgent.