MXS-1220: Move request body checks to a higher level

The checks whether a request body is present are now done at a higher
level. This removes the need to do the checks at the resource handler
callback, removing duplicated code.

The checks are done by adding constraints to resources that must be
fulfilled by each request.

Added debug assertions to make sure that the core logic of the REST API
resource system works.

server/core/maxscale/resource.hh

@@ -37,6 +37,12 @@ class Resource
     Resource& operator = (const Resource&);
 public:
 
+    enum resource_constraint
+    {
+        NONE         = 0,
+        REQUIRE_BODY = (1 << 0)
+    };
+
     Resource(ResourceCallback cb, int components, ...);
     ~Resource();
 
@@ -58,13 +64,28 @@ public:
      */
     HttpResponse call(const HttpRequest& request) const;
 
+    /**
+     * Add a resource constraint
+     *
+     * @param type Constraint to add
+     */
+    void add_constraint(resource_constraint type);
+
+    /**
+     * Whether resource requires a request body
+     *
+     * @return True if resource requires a request body
+     */
+    bool requires_body() const;
+
 private:
 
     bool matching_variable_path(const std::string& path, const std::string& target) const;
 
-    ResourceCallback        m_cb;   /**< Resource handler callback */
-    std::deque<std::string> m_path; /**< Path components */
-    bool                    m_is_glob; /**< Does this path glob? */
+    ResourceCallback        m_cb;          /**< Resource handler callback */
+    std::deque<std::string> m_path;        /**< Path components */
+    bool                    m_is_glob;     /**< Does this path glob? */
+    uint32_t                m_constraints; /**< Resource constraints */
 };
 
 /**