Disable SSLv3 (in favor of TLS) for the RPC client and server.

TLS is subject to downgrade attacks when SSLv3 is available, and
 SSLv3 has vulnerabilities.

The popular solution is to disable SSLv3. On the web this breaks
 some tiny number of very old clients. While Bitcoin RPC shouldn't
 be exposed to the open Internet, it also shouldn't be exposed to
 really old SSL implementations, so it shouldn't be a major issue
 for us to disable SSLv3.

There is more information on the downgrade attacks and disabling
 SSLv3 at https://disablessl3.com/ .
nit: Would it be better to initialize the contexts w/<code>ssl::context::tlsv1</code>?