Fix writeq_high_water and writeq_low_water

The parameters were never read at startup and could not be modified at
runtime. Also the values were only read once at startup.

include/maxscale/config.h

@@ -566,6 +566,15 @@ json_t* config_maxscale_to_json(const char* host);
  */
 uint32_t config_writeq_high_water();
 
+/**
+ * Set writeq high water mark
+ *
+ * @param size The high water mark in bytes
+ *
+ * @return True if the parameter was larger than MIN_WRITEQ_HIGH_WATER
+ */
+bool config_set_writeq_high_water(uint32_t size);
+
 /**
  * @brief  Get DCB write queue low water mark
  *
@@ -573,6 +582,15 @@ uint32_t config_writeq_high_water();
  */
 uint32_t config_writeq_low_water();
 
+/**
+ * Set writeq low water mark
+ *
+ * @param size The low water mark in bytes
+ *
+ * @return True if the parameter was larger than MIN_WRITEQ_LOW_WATER
+ */
+bool config_set_writeq_low_water(uint32_t size);
+
 /**
  * @brief Interpret a @disk_space_threshold configuration string.
  *