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