Simplify DCB writing code

Due to the changes in the threading model, the DCB write code can be
simplified by a great amount.

Since only one thread can write to a DCB, it's safe to assume that no new
data is added to the write queue of a DCB while it is being drained. This
removes the need for the code that tracks whether a concurrent DCB write
attempt was made.

Because the high and low water callbacks weren't used by any module, it is
safe to remove them. They offer no real benefits over the drain callback.

include/maxscale/dcb.h

@@ -187,8 +187,6 @@ typedef struct dcb
     skygw_chk_t     dcb_chk_top;
     bool            dcb_errhandle_called; /*< this can be called only once */
     bool            dcb_is_zombie;  /**< Whether the DCB is in the zombie list */
-    bool            draining_flag;  /**< Set while write queue is drained */
-    bool            drain_called_while_busy; /**< Set as described */
     dcb_role_t      dcb_role;
     DCBEVENTQ       evq;            /**< The event queue for this DCB */
     int             fd;             /**< The descriptor */
@@ -221,8 +219,6 @@ typedef struct dcb
     DCBMM           memdata;        /**< The data related to DCB memory management */
     DCB_CALLBACK    *callbacks;     /**< The list of callbacks for the DCB */
     long            last_read;      /*< Last time the DCB received data */
-    int             high_water;     /**< High water mark */
-    int             low_water;      /**< Low water mark */
     struct server   *server;        /**< The associated backend server */
     SSL*            ssl;            /*< SSL struct for connection */
     bool            ssl_read_want_read;    /*< Flag */