MXS-3106: Always write the final COM_QUIT

This seems to help with a regression that was introduced in 2.4.11.

include/maxscale/dcb.hh

@@ -206,6 +206,11 @@ struct DCB : public MXB_POLL_DATA
     bool           high_water_reached = false;  /** High water mark reached, to determine whether we need to
                                                  * release
                                                  * throttle */
+
+    // Enable this to prevent write errors from being logged. Helps reduce false errors when writes to
+    // potentially closed DCBs are done.
+    bool silence_write_errors = false;
+
     struct
     {
         DCB* next = nullptr;        /**< Next DCB in owning thread's list */

include/maxscale/protocol/mysql.hh

@@ -340,7 +340,6 @@ typedef struct
     bool                   collect_result;              /*< Collect the next result set as one buffer */
     bool                   changing_user;
     bool                   track_state;     /*< Track session state */
-    bool                   send_com_quit;
     uint32_t               num_eof_packets; /*< Encountered eof packet number, used for check
                                              * packet type */
     bool large_query;                       /*< Whether to ignore the command byte of the next