MXS-862: Move sending of OK packet to mysql_client

Moving the sending of the final OK packet of the authentication process to
the client protocol plugin makes the authentication plugins simpler.

By reading the client's sequence and incrementing that by one, the client
protocol module will always send the correct sequence byte in the final OK
packet.

server/modules/include/mysql_client_server_protocol.h

@@ -70,6 +70,10 @@
 #define MYSQL_HEADER_LEN 4L
 #define MYSQL_CHECKSUM_LEN 4L
 
+/** Offsets to various parts of the client packet */
+#define MYSQL_SEQ_OFFSET 3
+#define MYSQL_COM_OFFSET 4
+
 #define GW_MYSQL_PROTOCOL_VERSION 10 // version is 10
 #define GW_MYSQL_HANDSHAKE_FILLER 0x00
 #define GW_MYSQL_SERVER_CAPABILITIES_BYTE1 0xff