dhtproxy: add server two-ways tls

[binarytrails]

• run:
  tools/dhtnode --proxyserverssl 8080 --proxy-certificate ca.pem --proxy-privkey key.priv --proxy-privkey-password x --proxy-client-certificate client-certificates.pem -v

• in memory:
  
  It is possible to add in memory client certificates to the certificate authority store however, the fuction allowing it add_certificate_authority only takes one at time thus, if this is the way taken instead of using an external file containing many added with load_verify_file done at 92766f7, one should perhaps change the design to load other client certs dynamically or load many of them at run time with multiple args following --proxy-client-certificates.

[aberaud]

should be able to provide in-memory certificate (from shared_ptrcrypto::Certificate)

[binarytrails]

@aberaud we could use the in-memory feature to add a client certificate to the ssl strore [1] however, this would require design changes to update them dynamically on the server instance. An interesting thread explaining the underlying issue is here [2] and the solution to update them dynamically would be at [1]. To recall this feature is to be able to validate multiple clients with their certificates at the store.
[1] http://think-async.com/Asio/asio-1.12.2/doc/asio/reference/ssl__context/add_certificate_authority/overload2.html
[2] http://boost.2283326.n4.nabble.com/ASIO-ssl-load-verify-file-td2577261.html