Galexie: Add retry/backoff on resumability datastore requests

[chowbao]

If resumability encounters an error due to remote DataStore errors, during FindStart we should have a retry/backoff routine, with defaults, external config for tuning not necessary.

If after retrying the request x(threshold) times still fails we should propagate that error back to the caller.

for context, a full history data lake mayt require up to about 25 remote DataStore.Exists requests, so, to address possible transient errors due to remote link.