Fixes ThreadPool Exhaustion with ProgressListener

[TheAngryByrd]

When using FindAllReferences, I noticed it was taking extremely long to get results back. I noticed the logs coming in about every second which gave me a hunch that it might be threadpool exhaustion. I went digging and found a good article for debugging this sort of things.

Using dotnet-stack I was able to trace down the culprit:

Thread (0x19844):
  [Native Frames]
  System.Private.CoreLib.il!System.Threading.WaitHandle.WaitOneNoCheck(int32)
  FSharp.Core!Microsoft.FSharp.Control.AsyncPrimitives+ResultCell`1[System.__Canon].TryWaitForResultSynchronously(class Microsoft.FSharp.Core.FSharpOption`1<int32>)
  FSharp.Core!Microsoft.FSharp.Control.FSharpMailboxProcessor`1[System.__Canon].TryPostAndReply(class Microsoft.FSharp.Core.FSharpFunc`2<class Microsoft.FSharp.Control.FSharpAsyncReplyChannel`1<!!0>,!0>,class Microsoft.FSharp.Core.FSharpOption`1<int32>)
  fsautocomplete!<StartupCode$fsautocomplete>[email protected](class System.Diagnostics.Activity)
  System.Diagnostics.DiagnosticSource.il!System.Diagnostics.SynchronizedList`1[System.__Canon].EnumWithAction(class System.Action`2<!0,class System.Object>,class System.Object)
  System.Diagnostics.DiagnosticSource.il!System.Diagnostics.Activity.Start()
  System.Diagnostics.DiagnosticSource.il!System.Diagnostics.Activity.Create(class System.Diagnostics.ActivitySource,class System.String,value class System.Diagnostics.ActivityKind,class System.String,value class System.Diagnostics.ActivityContext,class System.Collections.Generic.IEnumerable`1<value class System.Collections.Generic.KeyValuePair`2<class System.String,class System.Object>>,class System.Collections.Generic.IEnumerable`1<value class System.Diagnostics.ActivityLink>,value class System.DateTimeOffset,class System.Diagnostics.ActivityTagsCollection,value class System.Diagnostics.ActivitySamplingResult,bool,value class System.Diagnostics.ActivityIdFormat,class System.String)
  System.Diagnostics.DiagnosticSource.il!System.Diagnostics.ActivitySource.CreateActivity(class System.String,value class System.Diagnostics.ActivityKind,value class System.Diagnostics.ActivityContext,class System.String,class System.Collections.Generic.IEnumerable`1<value class System.Collections.Generic.KeyValuePair`2<class System.String,class System.Object>>,class System.Collections.Generic.IEnumerable`1<value class System.Diagnostics.ActivityLink>,value class System.DateTimeOffset,bool,value class System.Diagnostics.ActivityIdFormat)
  FSharp.Compiler.Service!FSharp.Compiler.Diagnostics.Activity.start(class System.String,class System.Collections.Generic.IEnumerable`1<class System.Tuple`2<class System.String,class System.String>>)
  FSharp.Compiler.Service!<StartupCode$FSharp-Compiler-Service>[email protected](class Microsoft.FSharp.Core.Unit)
  FSharp.Compiler.Service!FSharp.Compiler.BuildGraph+Delay@45-2[System.__Canon].Invoke(class Microsoft.FSharp.Core.Unit)
  FSharp.Core!Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke(value class Microsoft.FSharp.Control.AsyncActivation`1<!!0>,!!1,class Microsoft.FSharp.Core.FSharpFunc`2<!!1,class Microsoft.FSharp.Control.FSharpAsync`1<!!0>>)
  FSharp.Core!Microsoft.FSharp.Control.Trampoline.Execute(class Microsoft.FSharp.Core.FSharpFunc`2<class Microsoft.FSharp.Core.Unit,class Microsoft.FSharp.Control.AsyncReturn>)
  FSharp.Core!<StartupCode$FSharp-Core>[email protected](class System.Object)
  System.Private.CoreLib.il!System.Threading.QueueUserWorkItemCallback.Execute()
  System.Private.CoreLib.il!System.Threading.ThreadPoolWorkQueue.Dispatch()
  System.Private.CoreLib.il!System.Threading.PortableThreadPool+WorkerThread.WorkerThreadStart()

Turns out I shouldn't be using PostAndReply as it blocks threads.