[core] Use std::mutex on RunLoop

Only the WorkTask mutex needs to be recusive because
the WorkQueue will run a WorkTask that will pop
itself from the WorkQueue causing it to get cancelled.

src/mbgl/util/run_loop.cpp

@@ -15,7 +15,7 @@ RunLoop::~RunLoop() {
 }
 
 void RunLoop::withMutex(std::function<void()>&& fn) {
-    std::lock_guard<std::recursive_mutex> lock(mutex);
+    std::lock_guard<std::mutex> lock(mutex);
     fn();
 }
 

src/mbgl/util/run_loop.hpp

@@ -146,7 +146,7 @@ private:
     void process();
 
     Queue queue;
-    std::recursive_mutex mutex;
+    std::mutex mutex;
     uv::async async;
 
     static uv::tls<RunLoop> current;