Change several *_index to *_id

[oxarbitrage]

In #110 we changed all the participant indexes to u64 and the full idea is to generate this identifiers in the dealer side(for the dealer-aggregator setup) #108

So, this are not indexes anymore but identifiers. We need to replace all the receiver_index, participant_index, etc names of the codebase into receiver_id, participant_id, etc.