Rebuild.php script uses array_walk incorrectly

Problem/Motivation

The code in rebuild.php to clear the user local caches has a couple of issues.

1. array_walk accepts the first array parameter by reference, so not passing a variable here generates warnings
2. array_walk passes the value as the first parameter and the key and the second to the callback function. So in PHP7 the call to apcu_clear_cache() is meant to have 0 parameters but it does not.

See https://3v4l.org/pS2ru for an example of this.

Proposed resolution

Use array_map instead, this was initially a problem in drush, see https://github.com/drush-ops/drush/pull/2451

Remaining tasks

User interface changes

n/a

API changes

n/a

Data model changes

n/a