Description:
------------
The session locking introduced with pecl-memcache-3.0.4 uses a steadily 
increasing wait time (from 5ms up to 1000ms) until the session is unlocked.

When sending several parallel AJAX requests (or lots in fast succession), you 
often end up at the max waiting time of 1000ms. This could make apps somewhat 
unresponsive if they are heavily AJAX dependent.

@see #22462
@see #58726

Test script:
---------------
server-side:

<?php
  $session = session_start();
  usleep(100000); // sleep 100 ms
?>

client-side:

<script language="text/javascript">
for (var i = 0; i < 10; i++) {
  xmlhttp=new XMLHttpRequest();
  xmlhttp.open("GET","http://localhost/session_lock.php?"+i,true);
  xmlhttp.send();
}
</script>

Expected result:
----------------
In an optimal case (and without using session_write_close()) each of the 10 AJAX 
requests would be executed once the previous one is finished. Therefore taking 
around 1.x seconds for all 10 requests.

Actual result:
--------------
Due to the increasing locking wait times the total time taken is far beyond that 
(depending on the server you could expect 5s total).