The sound of water tumbling over an 18th-century mill wheel could almost drown out the disbelief: a 2,150-acre horse farm in upstate Columbia County has just hit the market for a stratospheric $90 million. The property — known as Mill Farm — sprawls across meadows, barns and waterfalls in the hamlet of Ancramdale, roughly a 100 miles north of Manhattan. Its owner, former Bear Stearns financier Daniel Slott, pieced it together over 43 years. “What would be great for the community is if it were