SSL.ServerConnection: Improved protocol fallback.

Some cipher suites have been removed in later versions of TLS.
If the client and server versions both were recent, but had
no common ciphers that weren't removed in the negotiated
version of TLS, the handshake used to fail. Now we instead
try downgrading to older versions of TLS to see if there's
a common implemented suite then.

This fixes one of the common reasons for clients attempting
protocol downgrades on connection failure.