R1:02e1440ef260

Dump tables one at a time, rather than all at once

Summary:
Ref T13000. This allows us to be more selective about which tables we dump data for, to reduce the size of backups and exports. The immediate goal is to make large `ngrams` tables more manageable in the cluster, but this generally makes all backups and exports faster and easier.

Here, tables are dumped one at a time. A followup change will sometimes add the `--no-data` flag, to skip dumping readthrough caches and (optionally) rebuildable indexes.