Fix possible concurrency issue with the session mutex

The sessions vector may be changed concurrently with sessionCount()
being called. Thread sanitizer catches this issue.

sessionCount() is only used in session created/session destroyed log
messages, so I don't really think there's a big issue right now, and
the lock does of course come with some cost.

Author: RockinRoel

src/web/WebController.C

@@ -164,6 +164,9 @@ Configuration& WebController::configuration()
 
 int WebController::sessionCount() const
 {
+#ifdef WT_THREADED
+  std::unique_lock<std::recursive_mutex> lock(mutex_);
+#endif
   return sessions_.size();
 }
 

src/web/WebController.h

@@ -172,7 +172,7 @@ class WT_API WebController
 #ifdef WT_THREADED
   // mutex to protect access to the sessions map and plain/ajax session
   // counts
-  std::recursive_mutex mutex_;
+  mutable std::recursive_mutex mutex_;
 
   SocketNotifier socketNotifier_;
   // mutex to protect access to notifier maps. This cannot be protected