MXS-862: Refactor backend authentication handling

The backend responses are now read in one place and the functions just
read the data. The protocol level will now handle the packet gathering
process and the authentication part just inspects the data.

Backend connections now load authenticators when they are being
connected. In the future, this enables the use of authentication modules
for backend connection.

server/include/gw_authenticator.h

@@ -77,6 +77,29 @@ typedef struct gw_authenticator
 #define MXS_AUTH_LOADUSERS_OK    0 /**< Users loaded successfully */
 #define MXS_AUTH_LOADUSERS_ERROR 1 /**< Failed to load users */
 
+/**
+ * Authentication states
+ *
+ * The state usually goes from INIT to CONNECTED and alternates between
+ * MESSAGE_READ and RESPONSE_SENT until ending up in either FAILED or COMPLETE.
+ *
+ * If the server immediately rejects the connection, the state ends up in
+ * HANDSHAKE_FAILED. If the connection creation would block, instead of going to
+ * the CONNECTED state, the connection will be in PENDING_CONNECT state until
+ * the connection can be created.
+ */
+typedef enum
+{
+    MXS_AUTH_STATE_INIT, /**< Initial authentication state */
+    MXS_AUTH_STATE_PENDING_CONNECT,/**< Connection creation is underway */
+    MXS_AUTH_STATE_CONNECTED, /**< Network connection to server created */
+    MXS_AUTH_STATE_MESSAGE_READ, /**< Read a authentication message from the server */
+    MXS_AUTH_STATE_RESPONSE_SENT, /**< Responded to the read authentication message */
+    MXS_AUTH_STATE_FAILED, /**< Authentication failed */
+    MXS_AUTH_STATE_HANDSHAKE_FAILED, /**< Authentication failed immediately */
+    MXS_AUTH_STATE_COMPLETE /**< Authentication is complete */
+} mxs_auth_state_t;
+
 /**
  * The GWAUTHENTICATOR version data. The following should be updated whenever
  * the GWAUTHENTICATOR structure is changed. See the rules defined in modinfo.h