Prevent block inside ResolveAsync from blocking PollingResolver.Refresh

[JamesNK]

Addresses #2380 (comment)

User code is called inside a lock in the Refresh method. If the user code blocks without returning a task then the lock blocks future calls to Refresh.

The fix is to ensure user code is async by calling it from Task.Run.

[krasin-ga]

Looks good. I was also thinking about a scenario like this, but they both have the same root cause.

First thread

LockingPollingResolver.SomeCallback  🟢 lock (LockingPollingResolver._lock){base.Listener(endpoints)} 
  ConnectionManager.OnResolverResult  🔴 lock(ConnectionManager._lock){LoadBalancer.UpdateChannelState}
   ...

Second thread

SocketConnectivitySubchannelTransport.TryConnectAsync 
 Subchannel.UpdateConnectivityState 
   ConnectionManager.OnSubchannelStateChange 🟢 lock(ConnectionManager._lock){ subchannel.RaiseStateChanged(state, status);} 
      ChildHandlerController.RefreshResolver() 
       PollingResolver.Refresh() 
        PollingResolver.ResolveNowAsync() 
          LockingPollingResolver.ResolveAsync 🔴 lock(LockingPollingResolver._lock)base.Listener()} 
            ConnectionManager.OnResolverResult 
              lock(ConnectionManager._lock){LoadBalancer.UpdateChannelState}