FDB throws “The last backup happened in the future” when a backup is re-triggered from a different timezone

The error output is:

```
2020/12/10 07:58:01 ERROR: The last backup `2020-12-10-12-33-01.722470' happened in the future.
ERROR: An error was encountered during submission
Fatal Error: Backup error
```
The code here seems to be the offending part - https://github.com/apple/foundationdb/blob/master/fdbclient/FileBackupAgent.actor.cpp#L3835

We probably need to store the backup time as epoch, or with timezone info.

Read fdbclient/FileBackupAgent.actor.cpp around line 3835 and trace how the last-backup timestamp is stored and compared when a backup is re-triggered from another timezone. Reproduce the timezone change scenario, then verify that the backup no longer reports a future timestamp and completes successfully.

Written by the indexing model from the issue text.