Add reconfiguration to router API

Added a new router API entry point that allows configuration changes after
the instance has been created. This makes alterations to most service
parameters at runtime possible.

An option to reconfiguration would have been the creation of a new
service and the eventual destruction of the old one. This would be a more
complicated and costly method but from an architectural point of view it
is interesting.

The actual implementation of the configuration change is left to the
router. Currently, only readwritesplit performs reconfiguration as
implementing it with versioned configurations is very easy.

Versioned configurations can be considered an adequate first step but it
is not an optimal solution as it causes a bottleneck in the reference
counting of the shared configuration. Thread-specific configuration
definitions would make for a more efficient solution but the
implementation is more complex.

include/maxscale/router.h

@@ -211,6 +211,28 @@ typedef struct mxs_router_object
      * @param instance Router instance
      */
     void     (*destroyInstance)(MXS_ROUTER *instance);
+
+    /**
+     * @brief Configure router instance at runtime
+     *
+     * This function is guaranteed to be called by only one thread at a time.
+     * The router must declare the RCAP_TYPE_RUNTIME_CONFIG in its capabilities
+     * in order for this function to be called.
+     *
+     * Modifications to the router should be made in an atomic manner so that
+     * existing sessions do not read a partial configuration. One way to do this
+     * is to use shared pointers for storing configurations.
+     *
+     * @param instance Router instance
+     * @param params   Updated parameters for the service. The parameters are
+     *                 validated before this function is called.
+     *
+     * @return True if reconfiguration was successful, false if reconfiguration
+     *         failed. If reconfiguration failed, the state of the router
+     *         instance should not be modified.
+     */
+    bool (*configureInstance)(MXS_ROUTER *instance, MXS_CONFIG_PARAMETER* params);
+
 } MXS_ROUTER_OBJECT;
 
 /**
@@ -218,7 +240,7 @@ typedef struct mxs_router_object
  * must update these versions numbers in accordance with the rules in
  * modinfo.h.
  */
-#define MXS_ROUTER_VERSION  { 3, 0, 0 }
+#define MXS_ROUTER_VERSION  { 3, 1, 0 }
 
 /**
  * Specifies capabilities specific for routers. Common capabilities
@@ -231,10 +253,11 @@ typedef struct mxs_router_object
  */
 typedef enum router_capability
 {
-    RCAP_TYPE_NO_RSESSION   = 0x00010000, /**< Router does not use router sessions */
-    RCAP_TYPE_NO_USERS_INIT = 0x00020000, /**< Prevent the loading of authenticator
+    RCAP_TYPE_NO_RSESSION    = 0x00010000, /**< Router does not use router sessions */
+    RCAP_TYPE_NO_USERS_INIT  = 0x00020000, /**< Prevent the loading of authenticator
                                              users when the service is started */
-    RCAP_TYPE_NO_AUTH       = 0x00040000, /**< No `user` or `password` parameter required */
+    RCAP_TYPE_NO_AUTH        = 0x00040000, /**< No `user` or `password` parameter required */
+    RCAP_TYPE_RUNTIME_CONFIG = 0x00080000, /**< Router supports runtime cofiguration */
 } mxs_router_capability_t;
 
 typedef enum