Backup/restore: export and import through the store #748

Design PR 13 (#396, Phase 2): ExportTableToPointInTime as a consistent snapshot through open_table_snapshot into the store's exports/ component, asynchronous, with DescribeExport and ListExports; ImportTable from a store prefix, with DescribeImport and ListImports over the same job rows; ValidationException for ExportTime and incremental export instead of ignoring them. Closes #57 and supersedes #80 (its REPEATABLE READ snapshot shipped in PR 5, its keyset paging in PR 8). Design invariants: 5.4; exports/ is skipped by sync and left by purge. Documentation: the differences doc's Import and Export rows.

Part of #624. The design's PR 13. Depends on PR 9 (#740) and PR 10.