Revert "Run health checks asynchronously"

... because the code didn't work at all.

This reverts commit d0ad7f1fce50d96d6da669d2d719ef74ee5f0536.
Increase acceptable drift between STH and logs

This increase is because drift is expected, until
https://github.com/certificate-transparency-watch/hs-certificate-transparency/issues/19
is fixed.