Description:
------------
When two or more concurrent scripts try to save data to the APC user cache, and 
APC memory usage is high (near limit set by apc.shm_size option), function 
apc_store() randomly fails to save data and returns false. 

I`ve updated APC to latest available version from trunk (3.1.15-dev).

I provide simple test script. To reproduce the bug, you should:
1) Set apc.shm_size to relative small value (eg. 32M)
2) Leave the rest of options at default values
3  Run test script simultaneously in two different browsers. This is very 
important to have at least two instances executing at a time, because one single 
script will always work correctly. If the problem doesn`t occur, try to repeat 
this several times and make sure you have reached full memory usage allowed by 
limit (one execution of a script takes approximately 12MB of RAM).


Test script:
---------------
<?php 

for ($i = 0; $i < 20000; $i++) {
    $unique = uniqid();
    if (false === apc_store($unique, $unique, 3600)) {
        throw new Exception('Failed');
    }
    usleep(1000);
}
echo 'OK';


Expected result:
----------------
Script run in each browser should write values to the cache and display OK.

Actual result:
--------------
Result is correct only when APC has a lot of free memory available. 
When memory usage is high (near limit set by apc.shm_size) one of the scripts 
very often throws an exception, because apc_store() is unable to write to the 
cache. 

Test case works well with relatively small value of apc.shm_size, but the problem 
remains also on production servers, with 512MB or more shm_size.