implemented Cache class for read only API
implemented update of cached API answers
adoption of Cache for public API in /node
update of Cache after wizard is complete
minor refactor in order to have less races and eventually log them
added unit tests for GLApiCache
fixed bug related to GLApiCache and https://s3.amazonaws.com/archive.travis-ci.org/jobs/36482622/log.txt

when we add/update/delete a context or a receiver, both the receivers cache and
the contexts cache must be updated as well.
Merge pull request #225 from globaleaks/D8-OTF-Cache

D8-OTF cache of public API