Avoid overlapping timers (#535)

This fix avoids starting another callback until the current one has finished to avoid thread pool starvation.

We've automatically captured dumps where overlapping timers are all block on the same thread:

  ntdll.dll!_ZwWaitForMultipleObjects@20() Line 825 Unknown
  KERNELBASE.dll!WaitForMultipleObjectsEx(unsigned long nCount, void * const * lpHandles, int bWaitAll, unsigned long dwMilliseconds, int bAlertable) Line 1551 C
  [Managed to Native Transition]
  mscorlib.dll!System.Collections.Concurrent.ConcurrentDictionary<EnvDTE.Project, System.IO.FileSystemWatcher>.AcquireLocks(int fromInclusive, int toExclusive, ref int locksAcquired) Unknown
  mscorlib.dll!System.Collections.Concurrent.ConcurrentDictionary<EnvDTE.Project, System.IO.FileSystemWatcher>.AcquireAllLocks(ref int locksAcquired) Unknown
  mscorlib.dll!System.Collections.Concurrent.ConcurrentDictionary<EnvDTE.Project, System.IO.FileSystemWatcher>.GetValues() Unknown
> BundlerMinifierVsix.dll!BundlerMinifierVsix.Commands.ProjectEventCommand.TimerElapsed(object state) Unknown
  mscorlib.dll!System.Threading.TimerQueueTimer.CallCallbackInContext(object state) Unknown
  mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Unknown
  mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Unknown
  mscorlib.dll!System.Threading.TimerQueueTimer.CallCallback() Unknown
  mscorlib.dll!System.Threading.TimerQueueTimer.Fire() Unknown
  mscorlib.dll!System.Threading.TimerQueue.FireQueuedTimerCompletion(object state) Unknown
  mscorlib.dll!System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() Unknown
  mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch() Unknown
  mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() Unknown

In some dumps we saw over 1800 threads with the above stack.

src/BundlerMinifierVsix/Commands/ProjectEventCommand.cs

@@ -37,7 +37,7 @@ private ProjectEventCommand(IServiceProvider provider)
             _events.ProjectAdded += EnsureProjectIsActive;
             _events.ProjectRemoved += OnProjectRemoved;
 
-            _timer = new Timer(TimerElapsed, null, 0, 250);
+            _timer = new Timer(TimerElapsed, null, 0, Timeout.Infinite);
         }
 
         public static ProjectEventCommand Instance { get; private set; }
@@ -207,6 +207,11 @@ void TimerElapsed(object state)
             {
                 Logger.Log(ex);
             }
+            finally
+            {
+                // Only now fire off next event to avoid overlapping timers
+                _timer.Change(250, Timeout.Infinite);
+            }
         }
 
         class QueueItem