Clean up threading

* Rename everything.
    * Strip J prefix.
    * Change UpperCamelCase functions to lowerCamelCase.
  * Remove global (!) semaphore count mutex on OSX.
  * Remove semaphore count getter (unused, unsafe, depended on internal
    API functions on Windows, and used a hack on OSX).
  * Add `Atomic<type>`.
  * Make `Thread` handle thread names.
  * Add support for C++11 multi-threading.
  * Combine pthread and win32 sources.
  * Remove `ThreadStarted` (unused, unneeded).
  * Move some includes from the headers to the sources.
  * Move all of `Event` into its header (allows inlining with no new includes).
  * Make `Event` use `Semaphore` (except on Windows).
  * Move some porting functions into `Thread`.
  * Integrate logging with `Thread`.
  * Add threading test.

src/server.h

@@ -516,7 +516,7 @@ private:
 
 	// Environment
 	ServerEnvironment *m_env;
-	JMutex m_env_mutex;
+	Mutex m_env_mutex;
 
 	// server connection
 	con::Connection m_con;
@@ -557,7 +557,7 @@ private:
 	// A buffer for time steps
 	// step() increments and AsyncRunStep() run by m_thread reads it.
 	float m_step_dtime;
-	JMutex m_step_dtime_mutex;
+	Mutex m_step_dtime_mutex;
 
 	// current server step lag counter
 	float m_lag;