For 20 years, Anna Bissonnette has been a force to be reckoned with when it comes to elder homelessness. As a community nurse, Bissonnette, now 75, of Waltham could not walk away from the patients she was treating in various Boston neighborhoods. It was the mid-1980s and seniors in the 70s and 80s were being forced out of their homes and into homeless shelters by gentrification.