Simplify the recheckCAA function. (#3701)
Simplify the recheckCAA function. (#3701)

Using both a sync.WaitGroup and channel is somewhat unnecessary - instead
synchronize directly on the channel. Additionally, strings in Go are
immutable - as such using string concatentation in a loop results in
reallocation. Use a string slice and strings.Join, which not only avoids
this, but also cuts down on the lines of code needed.
lucasallan/boulder,jsha/boulder,lucasallan/boulder,jfrazelle/boulder,letsencrypt/boulder,lucasallan/boulder,jfrazelle/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder,bmw/boulder,jfrazelle/boulder,bmw/boulder,jcjones/boulder,letsencrypt/boulder,bmw/boulder,jcjones/boulder,jfrazelle/boulder