Avoid loading all terms on the taxonomy overview form

Problem/Motivation

When a taxonomy vocabulary contains too many terms, it is absolutely impossible to reach some pages like taxonomy overview, even if their content is paginated, because of huge memory usage.

Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 20480 bytes) in /var/www/html/web/core/lib/Drupal/Core/Cache/DatabaseBackend.php on line 167

This is caused by the taxonomy_get_tree function for huge taxonomies, but for medium sized taxonomies, this is also caused by the fact that we never clean unused variables.

For information, with a 72000 terms taxonomy (what is not so many for big websites) :

$start = memory_get_usage(TRUE);
$tree = \Drupal::entityManager()->getStorage('taxonomy_term')->loadTree($vid); // 72000 terms
$used_memory = memory_get_usage(TRUE) - $start;
echo $used_memory; // ~ 242 MB

Proposed resolution

Unset loadTree result when it is not needed anymore. This will not save the life of huge websites, but this will definetely help medium ones to stay usables.

Refactor the implementation to use less memory in general and especially with pagination.

Remaining tasks

Post the patch

User interface changes

None

API changes

None

Beta phase evaluation

-->