Safely acquire lock on multiple tables

[rkrage]

This functionality will allow us to improve the safety of operations that take out locks on multiple tables (e.g. adding / removing foreign keys).

I also discovered a bug in the nested lock acquisition logic, so this PR fixes that and adds more tests.

I feel like it's tradition at this point for me to include too many changes in a PR and for @jcoleman to ask me to break it up into smaller commits 😆 (~~I will try to do that tomorrow~~ done).