Use --hex-blob flag in bin/storage dump

Summary: mysqldump output can end up having weird encoding issues when raw BLOBs are in the output, preventing the backup restoration from succeeding. This hex-encodes blobs in the dump from the backup workflow causing the output file to only contain ASCII and ensure imports are successful.

Test Plan: Had issues restoring a backup from the original mysqldump command issued by this workflow. Ran the same command with this flag added and I was able to restore the backup.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11704