get_boundary_times | handle tables with dashes

[macbre]

2018-03-13 08:34:04 indexdigest.database.query          INFO     SELECT /* index-digest */ UNIX_TIMESTAMP(MIN(`created`)) as `min`, UNIX_TIMESTAMP(MAX(`created`)) as `max` FROM foo_bar-metadata
2018-03-13 08:34:04 indexdigest.database.query          ERROR    Database error #1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to