HeartbeatStream object is the object that takes stream
configuration to create a stream of activity objects.
It is the controlling organ at the pre-query, query and
post-query phases.
Abstract class HeartbeatStream
This base class has final template methods which are
used by the derived concretes. The HeartbeatStream is a
state object that is given to the HeartbeatStreamBuilder to
set the access to the current request.
