retriedWriteFile returns the supersession lookup's error instead of
treating an unanswered lookup as "not superseded": in a replay an older
snapshot could otherwise overwrite a newer remote object. The processor
retries the event. isSuperseded keeps its boolean form for the other call
sites. GetSyncOffset's KvGet carries a 30 s deadline so a startup that falls
back to the saved offset cannot hang on it.
TestGatewayUncachedRewriteIsStillUploaded asserted the upload on a failed
lookup; it now asserts the withheld upload and the lookup error.
(cherry picked from commit 8fc2e718f7cd8322615b4e40007be903ade2dbfa)
james
e4c0acfa61