Google Cloud Storage: Add exponential backoff on server errors

[gabrielreid]

Purpose

Brings the GCS implementation in line with the documented
expected behavior for dealing with HTTP 500 responses.

References

This PR resolves #1931

Ticket #1931 pointed to akka/akka#27461 as another potential resolution, but that ticket appears to have been discarded in the meantime.

Changes

• Exponential backoff is now used for waits of up to 32 seconds on 500 responses returned from GCS, as specified in the GCS SLA.

[ennru]

Great!
As this provides Sources, the use of a RestartSource makes total sense.
Just for the record, the RetryFlow ended up in Akka 2.6.0 in its simpler form in akka/akka#27742

I wonder if this calls for introducing settings so that the back-off can be configured by the user.

[gabrielreid]

I wonder if this calls for introducing settings so that the back-off can be configured by the user.

I was thinking about having these things in settings as well -- I decided against it (for now) because these specific timings are specified in the GCS SLA, so (in my view) they are part of the specification of the interface. I'm definitely open to another point of view on this though of course.

[ennru]

LGTM.

[ennru]

Oh, some trivial warnings make the build fail
https://travis-ci.com/akka/alpakka/jobs/258961188#L775

[gabrielreid]

Oh, some trivial warnings make the build fail
https://travis-ci.com/akka/alpakka/jobs/258961188#L775

Oops, I guess the Travis build runs with some flags that I didn't use locally, sorry about that.

I've fixed the warnings (I believe) and pushed a new version.

[ennru]

Thank you! A great improvement.