Consecutive calls to chrome.extension.sendMessage() - asynchronous

Question

I am writing an extension, where I want my own cookies store mycookies in background.js which stores domain, path, key and flags. In my background.js, when an HTTP response is received, I call chrome.extension.sendMessage() for each Set-Cookie header of the the response. So each response call this function many times within few m-seconds. After the response, only the last one or two cookies are stored, the rest are lost.
I did the following experiment. I connected a button with the function update_mycookies. A click on button calls the function immediately and displays the content of mycookies after 2500ms. If I read the store after 2500ms, all four cookies are there if I introduce the delays (as below) in update_mycookies but if I remove the delays, only the last one or two cookies are updated, the rest are lost. The chrome.extension.sendMessage is using `chrome.storage.local.set(). I want my store must be updated with all the cookies in response.
function update_mycookies(){
chrome.extension.sendMessage({"cdomain":".google.com", "path":"/", "key":"PREF"});
setTimeout(function() {
chrome.extension.sendMessage({"cdomain":".google.com", "path":"/", "key":"NID"});}, 1000);
setTimeout(function(){
chrome.extension.sendMessage({"cdomain":".google.it", "path":"/", "key":"PREF"}); }, 1500);
setTimeout(function(){
chrome.extension.sendMessage({"cdomain":".google.it", "path":"/", "key":"NID"}); }, 2000);
}
The onMessage listenr is the following:
chrome.extension.onMessage.addListener(
function(request,sender,sendResponse) {
if (request["cdomain"] && request["path"] && request["key"]) {
chrome.storage.local.get("mycookies", function (items) {
if (items["mycookies"]) {
var found = false;
var entries = items["mycookies"].split(";");
entries.forEach (function (entry) {
var dk = entry.split(",");
if (dk[0] == request["cdomain"] && dk[1] == request["path"] && dk[2] == request["key"])
found = true;
});
if (!found)
chrome.storage.local.set({"mycookies": items["mycookies"] + ";" + request["cdomain"] + "," + request["path"] + ","
+ request["key"] });
}
else
chrome.storage.local.set({"mycookies": request["cdomain"] + "," + request["path"] + "," + request["key"] });
});
}
}