Heartbleed - server response parsing

The original PR was here: https://github.com/rapid7/metasploit-framework/pull/3300 (see this PR for discussion on the module)
This PR readds the multiple connections since it's more reliable when dumping private keys.
The main benefit of this change is that you can now use the keydumping on the TLS_CALLBACK methods too.
Thanks to @jjarmoc for his help on this.