Move RWBackend into a separate file

Moved the RWBackend class implementation into its own file. Made some of
the command type functions a part of the <maxscale/protocol/mysql.h>
header to make it reusable.

include/maxscale/protocol/mysql.h

@@ -567,6 +567,15 @@ bool mxs_mysql_is_result_set(GWBUF *buffer);
  */
 bool mxs_mysql_is_prep_stmt_ok(GWBUF *buffer);
 
+/**
+ * Is this a binary protocol command
+ *
+ * @param cmd Command to check
+ *
+ * @return True if the command is a binary protocol command
+ */
+bool mxs_mysql_is_ps_command(uint8_t cmd);
+
 /**
  * @brief Check if the OK packet is followed by another result
  *