Implement key exchange mechanism to exchange seed of MAM

[howjmay]

DH agreement allows TA and its clients to share the same seed without unveiling the seed or transmitting it.
We need to implement DH agreement for MAM.

[howjmay]

The spec of DH agreement standard is here:
https://tools.ietf.org/html/rfc2631

[howjmay]

The shared secrect number can be used as the cryptographic seed which can generate IOTA seed.

1. We could use some usual cryptographic algorithm to hash the initial shared secrect
2. Then we can use Curl (IOTA hash function) to hash the result from the first stage into the IOTA seed in assigning length.

[howjmay]

There is implementaiton in mbedtls. Here is the source code that genrates keys for DH agreement
https://github.com/ARMmbed/mbedtls/blob/master/programs/pkey/dh_genprime.c

[howjmay]

client: https://github.com/ARMmbed/mbedtls/blob/master/programs/pkey/dh_client.c
server: https://github.com/ARMmbed/mbedtls/blob/master/programs/pkey/dh_server.c

[howjmay]

To support this implementation, the thirt-party gitmodule mbedtls will be moved to root thirt-party

[howjmay]

We will use asymmetric encryption to exchange seed instead of DH agreement.

[howjmay]

ECDH and ECDSA will be used to construct station to station protocol

[howjmay]

The shared secret of ECDH would be used to generate AES key. In this way, even we found out we shared the secret with malicious attacker, the user can change to the other share secret to generate the other AES key.