Always set a thread name in FunctionScheduler

Summary:
The name for the scheduling thread is specified with `FunctionScheduler::setThreadName`, which is not always called (or may even be called too late!), in which case the thread name is inherited from thread that calls `start()`, leading to confusing attribution.

Use a default name instead, as we do in several other executor-like primitives.

Reviewed By: philippv, luciang

Differential Revision: D40797728

fbshipit-source-id: 2e3d2516b0dc2fd07b6c2783de30797c8c2d2f43

folly/experimental/FunctionScheduler.cpp

@@ -430,9 +430,7 @@ bool FunctionScheduler::shutdown() {
 void FunctionScheduler::run() {
   std::unique_lock<std::mutex> lock(mutex_);
 
-  if (!threadName_.empty()) {
-    folly::setThreadName(threadName_);
-  }
+  folly::setThreadName(threadName_);
 
   while (running_) {
     // If we have nothing to run, wait until a function is added or until we

folly/experimental/FunctionScheduler.h

@@ -369,7 +369,7 @@ class FunctionScheduler {
   // or when the FunctionScheduler is stopped.
   std::condition_variable runningCondvar_;
 
-  std::string threadName_;
+  std::string threadName_{"FuncSched"};
   bool steady_{false};
   bool cancellingCurrentFunction_{false};
 };