Handle grpc errors of type UNAVAILABLE and ABORTED

Valentin Davidrequested to merge
valentindavid/bst-1/retry-cas-calls into bst-1

This is a rebase of !1002 (closed) to bst-1

Requests with error UNAVAILABLE are retried directly. They are typically issue with setting connection which can happen when the server is slow at accepting new connections.

Requests with errors ABORTED are converted to temporary exceptions so that the scheduler can retry the task.

Fixes #815

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