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.

Test Plan: Compared a dump from master and from this branch, found them to be essentially identical. The new dump has a little more header information in each section. Verified each contains the same number of CREATE TABLE statements.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13000

Differential Revision: https://secure.phabricator.com/D18679