Kraken is odd about data history APIs, they offer trade data, but only 720 candles, and regardless of timeframe, they keep returning the same 720, so the workaround is to use `--dl-trades` and resample into candles. So added a EXCHG_SPECIFIC_OPTS var to handle this for Kraken-only if kraken is the exchange in the loop iteration.
Kraken also, less oddly, applies rate limits by IP and Pair, so I have added support for parallel downloads for Kraken which currently sets default to the number of cores of the machine but is configurable via PARALLEL_REQS_KRAKEN