Discovery of N-[(3r)-1-Azabicyclo[2.2.2]oct-3-Yl]furo[2, 3-C]pyridine-5-Carboxamide, an Agonist of the A7 Nicotinic Acetylcholine Receptor, for the Potential Treatment of Cognitive Deficits in Schizophrenia: Synthesis and Structure-Activity Relationship