MXS-1220: Add Last-Modified and ETag headers

The resource system now tracks both the time when a resource was last
modified and the revision number of the resource. This allows working
Last-Modified and ETag headers to be generated by the REST API.

The If-Modified-Since and If-None-Match request headers are not yet
processed and using them will always return the resource instead of a 304
Not Modified response.

server/core/test/rest-api/test/http.js

@@ -0,0 +1,59 @@
+require("../utils.js")()
+
+
+describe("HTTP Headers", function() {
+    before(startMaxScale)
+
+    it("ETag changes after modification", function() {
+        return request.get(base_url + "/servers/server1", {resolveWithFullResponse: true})
+            .then(function(resp) {
+                resp.headers.etag.should.be.equal("0")
+                var srv = JSON.parse(resp.body)
+                delete srv.data.relationships
+                return request.put(base_url + "/servers/server1", {json: srv})
+            })
+            .then(function() {
+                return request.get(base_url + "/servers/server1", {resolveWithFullResponse: true})
+            })
+            .then(function(resp) {
+                resp.headers.etag.should.be.equal("1")
+            })
+    });
+
+    it("Last-Modified changes after modification", function() {
+        var date;
+
+        return request.get(base_url + "/servers/server1", {resolveWithFullResponse: true})
+            .then(function(resp) {
+
+                // Store the current modification time
+                resp.headers["last-modified"].should.not.be.null
+                date = resp.headers["last-modified"]
+
+                // Modify resource after three seconds
+                setTimeout(function() {
+                    var srv = JSON.parse(resp.body)
+
+                    srv.data.relationships = {
+                        services: {
+                            data: [
+                                {id: "RW-Split-Router", type: "services"}
+                            ]
+                        }
+                    }
+
+                    request.put(base_url + "/servers/server1", {json: srv})
+                        .then(function() {
+                            return request.get(base_url + "/servers/server1", {resolveWithFullResponse: true})
+                        })
+                        .then(function(resp) {
+                            resp.headers["last-modified"].should.not.be.null
+                            resp.headers["last-modified"].should.not.be.equal(date)
+                        })
+
+                }, 3000)
+            })
+    });
+
+    after(stopMaxScale)
+});