Fix reliability of caching algorithm (#40)
Fix reliability of caching algorithm (#40)

- perform checks every 900'000 milliseconds (i.e. 15 minutes)

- set immediately the `latestCheck` to prevent other requests that
  arrive when the update was started to trigger further updates

- respawn possibly failed child afer 15 seconds

- before listening attempt once to fetch the institutions
nexacenter/oa-check,nexacenter/oa-check,nexacenter/roarmap-h2020-view,nexacenter/roarmap-h2020-view