Shrink memory in threadpool (#21689)

### What does this PR do?

After 10s of inactivity in the thread pool, this releases memory more
aggressively back to the operating system

### How did you verify your code works?

src/threading/ThreadPool.zig

@@ -660,6 +660,7 @@ const Event = struct {
     noinline fn wait(self: *Event) void {
         var acquire_with: u32 = EMPTY;
         var state = self.state.load(.monotonic);
+        var has_shrunk_memory: bool = false;
 
         while (true) {
             // If we're shutdown then exit early.
@@ -699,7 +700,11 @@ const Event = struct {
             // Acquiring to WAITING will make the next notify() or shutdown() wake a sleeping futex thread
             // who will either exit on SHUTDOWN or acquire with WAITING again, ensuring all threads are awoken.
             // This unfortunately results in the last notify() or shutdown() doing an extra futex wake but that's fine.
-            Futex.wait(&self.state, WAITING, null) catch unreachable;
+            Futex.wait(&self.state, WAITING, if (!has_shrunk_memory) std.time.ns_per_s * 10 else null) catch {
+                has_shrunk_memory = true;
+                bun.Global.mimalloc_cleanup(false);
+                bun.jsc.wtf.releaseFastMallocFreeMemoryForThisThread();
+            };
             state = self.state.load(.monotonic);
             acquire_with = WAITING;
         }