perf (aggregator): send agg.sendAggregatedResponseToContract to go routine

[taturosati]

when calling agg.sendAggregatedResponseToContract we wait for response before subscribing back to channel. This limits the amount of responses we can send, since it is slow to submit response on chain. It should be sent to separate go routine so that aggregations can be handled in parallel.

An issue may arise with tx nonces and we may need to implement our own sequential nonces.