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 Could they be marked out if no longer applicable?