This PR implements a runtime heap allocation monitor which allows the application to watch the allocated heap size at regular intervals (which can be set and changed at will), with registered callback functions triggered any time the heap goes past each of a set of user-defined thresholds. The callback function is passed the threshold limit (which allows the same callback to be used for multiple thresholds) and the full set of runtime memory stats (not just the heap size), which the callback function may use to report memory telemetry to New Relic and/or take corrective action, as appropriate.