[gold] Close GCS reader in ingestion
[gold] Close GCS reader in ingestion

Maybe this is the/a cause of OOM and leaking goroutines?

Additionally, change the retry to use proper exponential backoff.

Change-Id: I621365f93f9fc468bdcf8894f044e510e196c4b4
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/220758
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot