Add tools for collecting data on message sending times.

Usage, on the javascript console:
> feature_flags.collect_send_times = true;
> compose.send_many_messages("test", "test", 3)
> compose.send_times_data
  [47, 99, 48]

(imported from commit a2a22eea9bb1bbe24a79ec0453c099dabfc61c19)

static/js/compose.js

@@ -345,8 +345,11 @@ function send_message_socket(request, success) {
     });
 }
 
-function send_message() {
-    var request = create_message_object();
+exports.send_times_data = [];
+function send_message(request) {
+    if (request === undefined) {
+        request = create_message_object();
+    }
     exports.snapshot_message(request);
 
     if (request.type === "private") {
@@ -360,6 +363,9 @@ function send_message() {
         if (feature_flags.log_send_times) {
             blueslip.log("send time: " + (new Date() - start_time));
         }
+        if (feature_flags.collect_send_times) {
+            exports.send_times_data.push((new Date() - start_time));
+        }
         $("#new_message_content").val('').focus();
         autosize_textarea();
         $("#send-status").hide(0);
@@ -380,6 +386,34 @@ function send_message() {
     }
 }
 
+// This function is for debugging / data collection only.  Arguably it
+// should live in debug.js, but then it wouldn't be able to call
+// send_message() directly below.
+exports.test_send_many_messages = function (stream, subject, count) {
+    var num_sent = 0;
+
+    function do_send_one () {
+        var message = {};
+        num_sent += 1;
+
+        message.type = "stream";
+        message.to = stream;
+        message.subject = subject;
+        message.content = num_sent.toString();
+        message.client = client();
+
+        send_message(message);
+
+        if (num_sent === count) {
+            return;
+        }
+
+        setTimeout(do_send_one, 1000);
+    }
+
+    do_send_one();
+};
+
 exports.finish = function () {
     clear_invites();
 

static/js/feature_flags.js

@@ -19,6 +19,7 @@ var is_customer4 = _.contains(customer4_realms, page_params.domain);
 
 // Manually-flipped debugging flags
 exports.log_send_times = false;
+exports.collect_send_times = false;
 
 // Permanent realm-specific stuff: