refactor: decouple worker threads from non-worker threads (#1137)

* Decouple workers.

* Moves code to separate file.

* Cleans up the exponential backoff.

* Initial working implementation.

* Refactors php threads to take callbacks.

* Cleanup.

* Cleanup.

* Cleanup.

* Cleanup.

* Adjusts watcher logic.

* Adjusts the watcher logic.

* Fix opcache_reset race condition.

* Fixing merge conflicts and formatting.

* Prevents overlapping of TSRM reservation and script execution.

* Adjustments as suggested by @dunglas.

* Adds error assertions.

* Adds comments.

* Removes logs and explicitly compares to C.false.

* Resets check.

* Adds cast for safety.

* Fixes waitgroup overflow.

* Resolves waitgroup race condition on startup.

* Moves worker request logic to worker.go.

* Removes defer.

* Removes call from go to c.

* Fixes merge conflict.

* Adds fibers test back in.

* Refactors new thread loop approach.

* Removes redundant check.

* Adds compareAndSwap.

* Refactor: removes global waitgroups and uses a 'thread state' abstraction instead.

* Removes unnecessary method.

* Updates comment.

* Removes unnecessary booleans.

* test

* First state machine steps.

* Splits threads.

* Minimal working implementation with broken tests.

* Fixes tests.

* Refactoring.

* Fixes merge conflicts.

* Formatting

* C formatting.

* More cleanup.

* Allows for clean state transitions.

* Adds state tests.

* Adds support for thread transitioning.

* Fixes the testdata path.

* Formatting.

* Allows transitioning back to inactive state.

* Fixes go linting.

* Formatting.

* Removes duplication.

* Applies suggestions by @dunglas

* Removes redundant check.

* Locks the handler on restart.

* Removes unnecessary log.

* Changes Unpin() logic as suggested by @withinboredom

* Adds suggestions by @dunglas and resolves TODO.

* Makes restarts fully safe.

* Will make the initial startup fail even if the watcher is enabled (as is currently the case)

* Also adds compareAndSwap to the test.

* Adds comment.

* Prevents panic on initial watcher startup.

testdata/fiber-basic.php

@@ -0,0 +1,9 @@
+<?php
+require_once __DIR__.'/_executor.php';
+
+return function() {
+    $fiber = new Fiber(function() {
+        echo 'Fiber '.($_GET['i'] ?? '');
+    });
+    $fiber->start();
+};