Use fewer keep-alives in VirtualEventBase::keepAliveRelease

Summary:
[Folly] Use fewer keep-alives in `VirtualEventBase::keepAliveRelease`.

{D5982132} mistakenly used `VirtualEventBase::add`, which holds a keep-alive on the master `EventBase`, instead of calling `EventBase::add` directly.

(Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D5992403

fbshipit-source-id: f427025c06f01e6a93d6eae8a051c3553f58be20

folly/io/async/VirtualEventBase.h

@@ -140,8 +140,8 @@ class VirtualEventBase : public folly::Executor, public folly::TimeoutManager {
   }
 
   void keepAliveRelease() override {
-    if (!inRunningEventBaseThread()) {
-      return add([=] { keepAliveRelease(); });
+    if (!evb_.inRunningEventBaseThread()) {
+      return evb_.add([=] { keepAliveRelease(); });
     }
     if (loopKeepAliveCountAtomic_.load()) {
       loopKeepAliveCount_ += loopKeepAliveCountAtomic_.exchange(0);