After HTTP framework disconnects, heartbeater idle-loops instead of being deleted.

In some cases, Master closes connection of HTTP framework without deleting the heartbeater:
https://github.com/apache/mesos/blob/65e18bef2c5ff356ef74bac9aa79b128c5b186d9/src/master/master.cpp#L3323
https://github.com/apache/mesos/blob/65e18bef2c5ff356ef74bac9aa79b128c5b186d9/src/master/master.cpp#L10910

It can be argued that this does not constitute a leak, because old heartbeaters are deleted on reconnection/removal.

However, this means that for each disconnected framework there is a ResponseHeartbeaterProcess that performs an idle loop.