Support new (1.7+) NodeChecks and ServiceChecks fields on sessions

[dantheoptimist]

As of Consul 1.7, the checks field has been deprecated and replaced with separate fields for node checks and service checks as per https://www.consul.io/api-docs/session

To support using these fields, these should be added to both the new session object and the session object for calls that return sessions with such checks present.