Added a build optional heartbeat pv, <suffix>:heartbeat, with value 0.
I updated the PCAS/casdef.h to add a new backwards compatiible virtual writeNotify interface in casChannel and casPV. This is important for the GW so that it can execute the proper form of ca_put or ca_put_callback in its implementation of write, or writeNotify, and so I did also install the necessary changes in the GW so that it will benefit from the new writeNotify interface.