Make EventHandler::isPending const

Summary: Because it just queries state

Test Plan: fbmake

Reviewed By: haijunz@fb.com

Subscribers: folly@lists

FB internal diff: D1332397

folly/io/async/EventHandler.cpp

@@ -156,7 +156,7 @@ void EventHandler::setEventBase(EventBase* eventBase) {
   eventBase_ = eventBase;
 }
 
-bool EventHandler::isPending() {
+bool EventHandler::isPending() const {
   if (event_.ev_flags & EVLIST_ACTIVE) {
     if (event_.ev_res & EV_READ) {
       return true;

folly/io/async/EventHandler.h

@@ -167,7 +167,7 @@ class EventHandler : private boost::noncopyable {
     return registerImpl(events, true);
   }
 
-  bool isPending();
+  bool isPending() const;
 
  private:
   bool registerImpl(uint16_t events, bool internal);