ticker: updates of markets.json are not atomic

[jacekn]

We noticed that occasionally GET requests for markets.json return empty result. The problem always corrects itself without intervention. It also always happens 2-3s after past a minute which makes me thing it's related with the following cronjob:

* * * * * /opt/stellar/bin/ticker generate market-data --db-url=xxx -o /opt/stellar/www/markets.json

I looked at the code and the file is truncated using os.Create() and then written again:
https://github.com/stellar/go/blob/master/services/ticker/internal/utils/main.go#L21

If there is a temporary storate slowness we can serve empty or incomplete file for a brief moment.

It would be good to make the update operation atomic (simple way could be to write markets.json.new and then move to markets.json but there may be a better way in go).

[accordeiro]

@jacekn thanks for finding the problem and already coming with a solution! :)