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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>