extmod/modbluetooth: Replace def_handle with end_handle in char IRQ.

This is technically a breaking change, but:
a) We need the end handle to do descriptor discovery properly.
b) We have no possible use for the existing definition handle in the
characteristic result IRQ. None of the methods can use it, and therefore
no existing code should be using it in a way that changing it to a
different integer value should break.

Unfortunately NimBLE doesn't make it easy to get the end handle, so also
implement a mechanism to use the following characteristic to calculate
the previous characteristic's end handle.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/library/bluetooth.rst

@@ -166,7 +166,7 @@ Event Handling
                 conn_handle, status = data
             elif event == _IRQ_GATTC_CHARACTERISTIC_RESULT:
                 # Called for each characteristic found by gattc_discover_services().
-                conn_handle, def_handle, value_handle, properties, uuid = data
+                conn_handle, end_handle, value_handle, properties, uuid = data
             elif event == _IRQ_GATTC_CHARACTERISTIC_DONE:
                 # Called once service discovery is complete.
                 # Note: Status will be zero on success, implementation-specific value otherwise.