Use real secret key management

[unito-bot]

Instead of https://ucsc-cgl.atlassian.net/browse/TOIL-312, we should make sure that any encryption keys we carry around are handled using a real library (that we didn’t write) that enforces an inability to log them or accidentally pickle them to the wrong place.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1682