Add support for identifying authentication messages

The pgprotocol overloads 'p' messages with PasswordMessage,
SASLInitialResponse, SASLResponse, and GSSResponse. This patch allows
contextual identification of the message by setting the authType in the
frontend and then setting this value in the backend when a
AuthenticationResponseMessage is received.

password_message.go

@@ -14,6 +14,9 @@ type PasswordMessage struct {
 // Frontend identifies this message as sendable by a PostgreSQL frontend.
 func (*PasswordMessage) Frontend() {}
 
+// Frontend identifies this message as an authentication response.
+func (*PasswordMessage) InitialResponse() {}
+
 // Decode decodes src into dst. src must contain the complete message with the exception of the initial 1 byte message
 // type identifier and 4 byte message length.
 func (dst *PasswordMessage) Decode(src []byte) error {