session: merge harness.go into session.go

The Session struct is defined in session.go — named after what it is.
No more 'harness' concept; that was an artifact of the interface+impl
pattern we removed.

session/waitchange_test.go

@@ -12,7 +12,7 @@ import (
 	olog "ollie/log"
 )
 
-// newTestCore returns a minimal harness for testing.
+// newTestCore returns a minimal Session for testing.
 func newTestCore(initialState string) *Session {
 	a := &Session{
 		id:    "test",