JNI: Fix TimeTag object reference in Device.Signal.release_instance().
JNI: Allow Device.Signal.release_instance() to be called with a default NOW timetag.
JNI: Add Device.Signal.update_instance() for all data types.

Note, as for Device.Signal.update(), we don't include `count'
parameter since it can be calculated from the array length and the
signal vector length.
JNI: Make TimeTag argument to Device.Signal.value() optional.
JNI: Add Device.Signal.instance_value().
JNI: Declare instance event type flags static.
JNI: Ensure memory for output signal object refs are initialized to zero.
JNI: Fix InstanceEventListener.onEvent method signature lookup.
JNI: rename Device.Signal.set_instance_event_callback() to set_instance_callback().
JNI: Add a test for the instance callback.
JNI: Add a check any time Device.Signal gets its mapper_signal pointer.
JNI: Don't assume value!=0 in input handler.
JNI: Create & pass in a TimeTag object during input handler.