HeartbeatAccess 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 heartbeataccess
This base class has final template methods which are
used by the derived concretes. The HeartbeatAccess is a
state object that is given to the HeartbeatMessageBuilder to
set the access to the current request.
