MINOR: checks: Use the check as origin when a session is created

MINOR: checks: Use the check as origin when a session is created

Before, the server was used as origin during session creation. It was only used
to get the check associated to the server when a variable is get or set in the
check scope or when a check sample fetch was called. So it seems easier to use
the check as origin of a session. It is also more logical becaues the session is
created by the health-check itself and not its server.