write behavior

[xresende]

currently a write to a writable pv automatically changes the pv value in the epics database.
but whether a value for the pv is valid or not has to be decided by the model that responds to the pv.

maybe we should change the behavior of the driver write function not to invoke setParam automatically. it should be invoked as a response to a send_queue os type 's' when the model has accepted the setpoint value. this proposed behavior seems to be closer to what real IOCs will do...