NEW YORK — Once upon a time, not too long ago, milkmen placed glass bottles full of milk on the doorsteps of homes and shops and cleared away the empties. In the early 1900s and throughout the first half of the 20th century, thousands of American dairies employed milkmen to deliver their goods to customers’ homes, first by horse and wagon, and later by truck. While a few dairies still offer some form of home delivery, the sight of a cheerful fellow in a crisply pressed uniform arriving at the front door has largely disappeared from our shared cultural memory, and the relics of that era have become coveted antiques.