MXS-862: Add create/destroy and remove plugin_name entry points

The create and destroy entry points allow authenticators to store data in
the DCB. This data is not shared by other DCBs related to the same
session.

The plugin_name entry point wasn't really useful as the plugins would
still need to send a AuthSwitchRequest packet if they wanted to change the
authentication mechanism.

server/modules/include/mysql_client_server_protocol.h

@@ -81,6 +81,9 @@
 
 #define DEFAULT_MYSQL_AUTH_PLUGIN "mysql_native_password"
 
+/** All authentication responses are at least this many bytes long */
+#define MYSQL_AUTH_PACKET_BASE_SIZE 36
+
 /** Maximum length of a MySQL packet */
 #define MYSQL_PACKET_LENGTH_MAX 0x00ffffff
 
@@ -297,6 +300,8 @@ typedef struct
 /* The following can be compared using memcmp to detect a null password */
 extern uint8_t null_client_sha1[MYSQL_SCRAMBLE_LEN];
 
+MYSQL_session* mysql_session_alloc();
+
 MySQLProtocol* mysql_protocol_init(DCB* dcb, int fd);
 void           mysql_protocol_done (DCB* dcb);
 const char *gw_mysql_protocol_state2string(int state);