waiting for lock in D7's variable_initialize() is optional and off by default

There's a lock on cache miss in D7's variable_initialize() and threads which failed to acquire this lock would wait, which is often unnecessary and can cause performance problems under high load / concurrency, especially if the variable system is being abused / frequently updated.

In Drupal 7.76 waiting on the lock became optional and off by default, which should typically be a performance improvement for most sites.

If this change causes issues for your site, you can revert to the original behaviour whereby threads which get a cache miss and fail to acquire the lock will wait. This can be done by setting the following variable in settings.php

$conf['variable_initialize_wait_for_lock'] = TRUE;