PB export: Implement exporting metadata.
PB export: Improved writing logic.
Now we are able to append to existing files and also check their consistency, and check they don't have samples newer than the new ones.
Still need to bound the query interval to the last timestamp in the output.
PB export: Refactoring: Move verification code.
PB export: Implement finding the last timestamp (but not yet bounding the query).
backend: Allow passing in raw time in (sec, nsec).
The current code always expected a datetime in local time.
PB export: Finish implementation of resumption.
PB export: Improve and restrict input time specification (explicit UTC only).