Fix Diffie Hellman key exchange

[ben-krieger]

1. The device's secret gets set to null in an erroneous call to destroy before it is used to compute the shared secret, thus causing modPow to throw an exception.
2. The DiffieHellman.KeyExchange type cannot reconstruct itself from an AnyType without a @JsonCreator-annotated constructor.
3. The shared secret is sometimes 1 extra byte in length, due to the behavior of BigInteger.toBytes(). See https://stackoverflow.com/questions/24158629/biginteger-tobytearray-returns-purposeful-leading-zeros.