ThreadPoolExecutor: throw in getPendingTaskCountImpl instead of pure virtual.

Summary:
Calling `folly::ThreadPoolExecutor::withAll` is powerfull when combined with `folly::ThreadPoolExecutor::getPoolStats`.
However getPendingTaskCountImpl being pure virtual can trigger `__cxa_pure_virtual ... std::terminate()` while object is partially destructed.

This change signals implementors of derived clases to write a body for getPendingTaskCountImpl while allowing calling getPoolStats -> getPendingTaskCountImpl within a try/catch block.

Reviewed By: yfeldblum

Differential Revision: D17915523

fbshipit-source-id: 2ffad7076313407969d88c75c7e457fa4e7e2d6d

folly/executors/ThreadPoolExecutor.h

@@ -225,7 +225,9 @@ class ThreadPoolExecutor : public DefaultKeepAliveExecutor {
   }
 
   // Prerequisite: threadListLock_ readlocked or writelocked
-  virtual size_t getPendingTaskCountImpl() const = 0;
+  virtual size_t getPendingTaskCountImpl() const {
+    throw std::logic_error("getPendingTaskCountImpl not implemented");
+  }
 
   class ThreadList {
    public: