Revert "Copy all statsd stats to Prometheus. (letsencrypt#2474)" (let…

…sencrypt#2541)

This reverts commit 58ccd7a.

We are seeing multiple boulder components restart when they encounter the stat registration race condition described in letsencrypt#2540

Merge pull request letsencrypt#2049 from letsencrypt/staging

Merge staging into release

Fix crash in expiration-mailer (letsencrypt#1997)

In letsencrypt#1923 we changed reg.Contact to a pointer, which can be nil if the corresponding data from the DB is the literal string "null". This causes panics in expiration-mailer, which we need to fix.

This change fixes modelToRegistration to always return a pointer to a non-nil slice. It also adds an extra sanity check in expiration-mailer itself.

Fixes letsencrypt#1993

letsencrypt#1997

Merge pull request letsencrypt#1232 from letsencrypt/caa-fix

Invalidate challenge on CAA failure