This patch adds support for a custom check id and name when
registering a service.
This is achieved by adding a CheckID and a Name field to the
CheckType structure which is used to register checks with a
service and when returning health check definitions.
CheckDefinition is a superset of CheckType which duplicates
some of the fields of CheckType. This patch decouples these
two structures by removing the embedding of CheckType in
CheckDefinition.
Fixes#3047
