Add an upstream ProxySQL container to our docker-compose. Configure
ProxySQL to manage database connections for our unit and integration
tests.
Fixes#5873
* Add cmd/expired-authz-purger with integration test
* Return count
* gofmt >.>
* add to boulder-config-next.json
* Commit missing file
* Exec on the dbMap
* fprintf the error message
* Review fixes + test
* Review fixes pt. 1
* Review fixes pt. 2 (actually add test file this time :|)
* Fix prompt
* Switch to using flag lib
* Use COUNT(1)
* Revert config -> flag stuff
* Review fixes
* Revert-revert COUNT(1) change
* Review fixes pt. 1
* Nest config struct
* Test review fixes
* Factor out getting future output with FAKECLOCK
* Review fixes pt. 2
* Review fixes pt. 3