ssl: rudimentary decryption for TLS 1.2

Several limitations still apply:
- TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 is the only supported cipher suite
- Some tests are broken due to a failing assertion regarding bytestring
- No newly written tests for decryption (the patch was tested extensively for our paper)
- Several small open technical questions marked with FIXME
- Architecture in the ssl module might not be optimal

testing/btest/scripts/base/protocols/ssl/tls13.test

@@ -37,7 +37,7 @@ event ssl_established(c: connection)
 	print "established", c$id;
 	}
 
-event ssl_encrypted_data(c: connection, is_orig: bool, record_version: count, content_type: count, length: count)
+event ssl_encrypted_data(c: connection, is_orig: bool, record_version: count, content_type: count, length: count, payload: string)
 	{
 	print "encrypted", c$id, is_orig, SSL::version_strings[record_version], content_type;
 	}