MXS-862: Add SQLite based authentication checks

The authentication checks make sure that a user has all the required
grants to access the database. This prevents the creation of unnecessary
backend connections reducing the overall load on the database.

Doing preliminary authentication in MaxScale enables the creation of more
informative error messages.

server/modules/authenticator/gssapi_auth.h

@@ -18,6 +18,7 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <gssapi.h>
+#include <sqlite3.h>
 
 MXS_BEGIN_DECLS
 
@@ -36,19 +37,16 @@ enum gssapi_auth_state
     GSSAPI_AUTH_FAILED
 };
 
-/** Common state tracking structure */
+/** Common structure for both backend and client authenticators */
 typedef struct gssapi_auth
 {
     enum gssapi_auth_state state; /**< Authentication state*/
     uint8_t *principal_name;      /**< Principal name */
     size_t principal_name_len;    /**< Length of the principal name */
     uint8_t sequence;             /**< The next packet seqence number */
+    sqlite3 *handle;              /**< SQLite3 database handle */
 } gssapi_auth_t;
 
-/** These functions can used for the `create` and `destroy` entry points */
-void* gssapi_auth_alloc(void *instance);
-void gssapi_auth_free(void *data);
-
 /** Report GSSAPI errors */
 void report_error(OM_uint32 major, OM_uint32 minor);