How to handle zstor temporary outage/SIGKILL

[OmarElawady]

If zdb calls the hook and zstor was down, nothing is done to make sure the datafile is uploaded. Also, a recent change is zstor was made to make the store commands non-blocking and it internally queues theses commands. If zstor was SIGKILLed, this data is never uploaded again.
After discussion with @LeeSmet @maxux, two approaches is suggested. To make zstor client store the store commands in a persistent queue from which zstor can pick up the commands and execute them. The second approach is to make zdb use the check zstor command to check that the files are uploaded successfully. For example, it can keep track of last uploaded data file, and check periodically the file after it for being successfully uploaded, and if it's not it can reissue the store command.

[maxux]

Commands issue in zdb hook are usually blocking (not all of them), it's better to have the fastest hook possible, adding some check and retry there is really not a good idea :)