GCP: Set Custom-Time in cached entries

[aSemy]

Sets the custom-time when adding or creating a cache entry, which will allow for a more optimal expiration policy.

Resolves #47

I'm not sure how to test this. Please could you help?

I also tidied up a few bits and pieces. E.g. formatting exception messages so they don't have newlines in the middle of the message, and using newer Kotlin features. Please let me know if this is a problem, and I should move them to another PR!

[liutikas]

This adds an extra HTTP call for every cache hit, this sounds like it will negatively affect the performance. It seems that rerunning a task that had expired the cache based on creation time is a somewhat reasonable approach does not penalize every cache hit.