By Caldwell Titcomb In “The Bartered Bride,” Jennifer Aylmer plays Marenka, who loves the farmhand Jenik, but is pressured to marry Vasek, the son of a wealthy neighbor. Boston has had the unusual luck of experiencing two major Czech operas within a few weeks. First, the Boston Lyric Opera gave us Antonin Dvořák’s “Rusalka” (see…