verify that IOThreadPools get shut down (#635)

Motivation:

Not shutting down IOThreadPools leads to thread leaks so we should
verify they're actually shut down.

Modifications:

add a check that we're shutting down

Result:

fewer thread leaks

Sources/NIO/BlockingIOThreadPool.swift

@@ -169,6 +169,15 @@ public final class BlockingIOThreadPool {
             }
         }
     }
+    
+    deinit {
+        switch self.state {
+        case .stopped, .shuttingDown:
+            ()
+        default:
+            assertionFailure("wrong state \(self.state)")
+        }
+    }
 }
 
 public extension BlockingIOThreadPool {