Switched to libuv mutexes

refs #137 (comment)

- we don't actually need to depend on Boost for mutexes, even though it
  was gated behind `NODE_SQLITE3_BOOST_THREADING` anyway, which I can't
  find documented anywhere
- this simplifies the threading.h file and keeps the different systems
  more aligned in their dependency use

src/async.h

@@ -6,11 +6,6 @@
 
 #include "threading.h"
 
-#if defined(NODE_SQLITE3_BOOST_THREADING)
-#include <boost/thread/mutex.hpp>
-#endif
-
-
 // Generic uv_async handler.
 template <class Item, class Parent> class Async {
     typedef void (*Callback)(Parent* parent, Item* item);

src/threading.h

@@ -1,48 +1,10 @@
 #ifndef NODE_SQLITE3_SRC_THREADING_H
 #define NODE_SQLITE3_SRC_THREADING_H
 
-
-#ifdef _WIN32
-
-#include <windows.h>
-
-    #define NODE_SQLITE3_MUTEX_t HANDLE mutex;
-
-    #define NODE_SQLITE3_MUTEX_INIT mutex = CreateMutex(NULL, FALSE, NULL);
-
-    #define NODE_SQLITE3_MUTEX_LOCK(m) WaitForSingleObject(*m, INFINITE);
-
-    #define NODE_SQLITE3_MUTEX_UNLOCK(m) ReleaseMutex(*m);
-
-    #define NODE_SQLITE3_MUTEX_DESTROY CloseHandle(mutex);
-
-#elif defined(NODE_SQLITE3_BOOST_THREADING)
-
-#include <boost/thread/mutex.hpp>
-
-    #define NODE_SQLITE3_MUTEX_t boost::mutex mutex;
-
-    #define NODE_SQLITE3_MUTEX_INIT
-
-    #define NODE_SQLITE3_MUTEX_LOCK(m) (*m).lock();
-
-    #define NODE_SQLITE3_MUTEX_UNLOCK(m) (*m).unlock();
-
-    #define NODE_SQLITE3_MUTEX_DESTROY mutex.unlock();
-
-#else
-
-    #define NODE_SQLITE3_MUTEX_t pthread_mutex_t mutex;
-
-    #define NODE_SQLITE3_MUTEX_INIT pthread_mutex_init(&mutex,NULL);
-
-    #define NODE_SQLITE3_MUTEX_LOCK(m) pthread_mutex_lock(m);
-
-    #define NODE_SQLITE3_MUTEX_UNLOCK(m) pthread_mutex_unlock(m);
-
-    #define NODE_SQLITE3_MUTEX_DESTROY pthread_mutex_destroy(&mutex);
-
-#endif
-
+#define NODE_SQLITE3_MUTEX_t uv_mutex_t mutex;
+#define NODE_SQLITE3_MUTEX_INIT uv_mutex_init(&mutex);
+#define NODE_SQLITE3_MUTEX_LOCK(m) uv_mutex_lock(m);
+#define NODE_SQLITE3_MUTEX_UNLOCK(m) uv_mutex_unlock(m);
+#define NODE_SQLITE3_MUTEX_DESTROY uv_mutex_destroy(&mutex);
 
 #endif // NODE_SQLITE3_SRC_THREADING_H