SSL.context: Select certificates in priority order.

When there are multiple certificates of the same type,
select the one with the strongest key if possible.
Replace almost unused KE_TO_SA with KE_Anonymous. Removed the SSL module internal SIGNATURE constants.
SSL.connection: Heartbeats now actually work.

Fixed multiple typos that caused the heartbeat code to fail.
SSL: Probe peer for the Heartbleed vulnerability.

The first packet sent after a successful handshake is now a
packet to probe for the Heartbleed (CVE-2014-0160) vulnerability,
and the connection is aborted with an insufficient_security
failure if the peer is suffering.

Also adds a debug flag (SSL3_SIMULATE_HEARTBLEED) to simulate
this state.