Chronic pain has invaded the bodies of every character in “Infinite Life,” a constant companion as they lounge in the sun, ruminating on their paths to a fasting clinic, sipping water, hungering for peace, conjuring distractions and supporting each other in their separate journeys. In a quietly compelling 110 minutes, the Annie Baker play now at barebones productions in Braddock, does not tick off any boxes of expectations; rather, it lives and breathes in a singular realm of slow-burn storytelling.