Move memory management from MigrateExecutable to an event subscriber

Re-roll for 10.1.x, as I'm interested in testing this out for a migration which is having memory issues.

Re-roll of 91 to account for CR-3159012: Symfony Event class deprecated, EventDispatcher::dispatch() argument order changed → .

And one more for coding standards.

All new properties need types and should use constructor promotion

1. +++ b/core/modules/migrate/src/MemoryManager.php
   @@ -0,0 +1,148 @@
   +   * @var float
   ...
   +  protected $memoryReclaimThreshold;
   ...
   +   * @var float
   ...
   +  protected $memoryThreshold;
   ...
   +   * @var int
   ...
   +  protected $memoryLimit;
   ...
   +   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
   ...
   +  protected $dispatcher;
   

   needs to be typed and could be part of constructor

2. +++ b/core/modules/migrate/src/MemoryManager.php
   @@ -0,0 +1,148 @@
   +   * @param int|string $memory_limit
   ...
   +  public function __construct(EventDispatcherInterface $dispatcher, $reclaim_threshold, $memory_threshold, $memory_limit = NULL) {
   

   int|string|null

Added type-hints to fix #99

It's often less arguments passed, so no reason in deprecation and event dispatcher also could be created in constructor to minimize amount of useless extra function calls (will file follow-up)

Here's a fix for CS and fix test - memory_usage always int, never null

To run tests the method is required, also fix one more place (test still fails)

Fix last broken test

Final polishing

Re-rolled last patch as MR

There are some bullets in the remaining tasks that can't tell if they still apply. Could they be marked out if no longer applicable?

tests pass

I've gone through and responded to most of the feedback mentioned in the IS. The requests in #80 & #82 are not super clear what is being asked given that the EnvironmentMemory object didn't materialize. Can we cross them off as well?

The LRU cache finally landed in Drupal core (see related issues). It is likely that migrate can remove its memory handling as a result.

Yes we might be able to delete this code altogether after 📌 Use LRU Cache for static entity cache Postponed .

Appears to need a rebase.

Wonder if the follow up tag is still needed?

If you are another contributor eager to jump in, please allow the previous poster at least 48 hours to respond to feedback first, so they have the opportunity to finish what they started!

As the issue 📌 Use LRU Cache for static entity cache Postponed is postponed it makes sense to file follow-up to clean-up and add TODO there, for #113

and needs rebase/reroll too