don't write the price for a coin if the coin hasn't moved

[Azulinho]

if we were to not write down into the price.log the price every second, but only when it changed we can save an considerable amount of disk space.

zcat coin.LUNETH.log.gz| uniq -f3|wc -l
135
 zcat coin.LUNETH.log.gz|wc -l
4670083

what we need to make sure, is that the 'timeouts' and backtesting 'process coins' process still work correctly, as for example the backtesting process will iterate over price.log lines, and as such timeouts wouldn't not be executed correctly if no lines are feed into that process.

[Azulinho]

fully done in #37