The current implementation of idle connection timeouts is not safe. The sessions
are handled in a way which is not thread-safe and the checking is done from
a non-polling thread.
With this change, the checks for the session timeouts are done in one of the
polling threads in a thread-safe manner only if at least one service has enabled
the timing out of idle client connections.
This keeps the behavior the same but allows the heartbeat to be used as a crude timer
without including everything the housekeeper.h header includes.
Moved the protocol level updates on the DCBs last_read value into dcb_read.
This keeps the implementation nice and compact.
