Switch to VirtualEventBase

Summary:
There's discussion on [this post](https://fb.prod.workplace.com/groups/560979627394613/permalink/2878701882289031/) where it was suggested to not use `runOnDestructionStart` since it special cases revproxy. Ideally that function would be removed since there's already a way to run some code on destruction start via a `VirtualEventBase`.

This diff moves us to do the above. Our shutdown logic is very confusing, but at a high level what we're trying to achieve is the following:
- On first SIGTERM, begin shutting down the workers. Have them continue looping until either:
  - All events have been processed
  - We've been force stopped via a second SIGTERM
- After all events have been process or we've been force stopped, let the main thread know. Once all workers have notified the main thread, we shut down the main thread.

Prior to these changes, the flow was:
- schedule a `cleanup` callback to `runOnDestructionStart`
- `cleanup` would `loop()` and also check if `forceStopped_` was set to true
- the worker would then call `callback_.workerFinished(this);`, which would notify the main thread that it was done processing events

Now the flow is as follows:
- we manage the event bases with a virtual event bases. This also will let us schedule a callback that will be run when destruction begins. However, **we cannot** call `loop` from the callback of a virtual event base, since the virtual event base loop is driven by the main event base, and you cannot call `loop` from an already-looping callback
- To address the above, we set `isWaitForAll(true)` on the executor, which will make each event base `loop()` to flush pending events before it exits (exactly as we want).

Reviewed By: HsiehYuho

Differential Revision: D55028184

fbshipit-source-id: d7c19269cdcb6cadf3c09414b337928ac3c3810b

Author: Keivaun Waugh

proxygen/lib/services/RequestWorkerThread.cpp

@@ -75,16 +75,11 @@ void RequestWorkerThread::setup() {
 }
 
 void RequestWorkerThread::forceStop() {
-  forceStopped_.store(true);
   evb_->terminateLoopSoon();
 }
 
 void RequestWorkerThread::cleanup() {
   LOG(INFO) << "Worker " << unsigned(getWorkerId()) << " in cleanup";
-  if (!forceStopped_.load()) {
-    LOG(INFO) << "Looping to finish pending work";
-    evb_->loop();
-  }
   callback_.workerFinished(this);
 }
 

proxygen/lib/services/RequestWorkerThread.h

@@ -125,8 +125,6 @@ class RequestWorkerThread {
   folly::EventBase* evb_{nullptr};
 
   static thread_local RequestWorkerThread* currentRequestWorker_;
-
-  std::atomic_bool forceStopped_{false};
 };
 
 } // namespace proxygen