Use --single-transaction in `bin/storage dump`

Summary: See <https://github.com/phacility/phabricator/issues/665>. From reading documentation, this seems dramatically better for InnoDB tables than the default behavior.

Test Plan: Ran `bin/storage dump`, got a reasonable-looking dump.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10606