When the client was started without any validators, the doppelganger
detection structures were never initialized properly. Later, when
validators were added through the Keymanager API, they interacted
with the uninitialized doppelganger detection structures and their
duties were inappropriately skipped.
* Keymanager API for the validator client
* Properly treat the 'description' field as optional when loading Keystores
* Spec-compliant serialization of the slashing data in Keymanager's DeleteKeys response ()
Fixes#3940Fixes#3964Closes#3884 by adding test