Add support for CHACHA20 TLS 1.2 ciphers

Our cipher suite preferences (which we get from Mozilla) include two cipher suites that we currently don't accept:

• TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
• TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

As far as we know, these cipher suites are not available for Java 8, but in theory a client preferring these ciphers should be able to fall back if they're unavailable.

If we decide this is a worthwhile investment, we should verify that everything works as expected for Java 8 (i.e. we fall back to an appropriate cipher suite and nothing blows up).