Cache for google-bucket stored files

[dvayanu]

When we use google buckets to store files the retrieval is pretty time intensive (and probably costly).
Instead of retrieving every time from the bucket, lets add a local cache (configured in the same file as the rest of bucket).

Case 1 - No file stored in cache.

• Retrieve file from google bucket and store it locally.
• Retrieve meta-information from the bucket and store it locally in the cache.
• Return the file

Case 2 - File stored in cache.

• Read and return file to user.
• Afterwards, in another executor, asynchronously, retrieve meta information about the file from bucket. If the file hasn't changed stop.
• If the file has changed retrieve and store both the file and the meta-information (same as 1).

Therefor the file will always be outdated once, but update by next request.