Always wrap DNSCryptoKeyEngine objects in a shared pointer

Always wrap DNSCryptoKeyEngine objects in a shared pointer

It's done almost everywhere, but not quite, and some of the paths
where it's not could leak if an exception is raised.
Also mark the overridden virtual methods with `override` to prevent
future mistakes.

(cherry picked from commit e69c2dac28d798813dd8e4a986c5045c63806ef0)