upgrade lua-resty-session to v4 and lua-resty-openidc to 1.8

[jtmkrueger]

resolves 18F/api.data.gov#697

There's a number of things that had to happen here in order to get these upgrades going

• remove all the plugins, since those are not a thing in v4. This required a good bit of refactoring
• update to new session api. This was a big refactor in v4. Had to change a number of things to accommodate.
• sessions table changed quite a bit. There's a migration here to update, and all the references in the code are touched.
• v4 creates its own pgmoon connections without search_path set, then returns them to the shared pool. When the code picked up those reused connections, it skipped re-running SET search_path (it only ran setup on fresh connections), causing stuff like "relation admins does not exist" errors. There's some refactoring to deal with this.
• cookies changed a lot. There's a number of changes to update to v4.

I tried to be verbose with comments in test/support/api_umbrella_test_helpers/admin_auth.rb, because it's kinda math-heavy. Leaned on built in libs as much as possible, but wanted to be clear about what's happening in that stuff.