[core] [node] pass thread pool impl to Map constructor

Updates mbgl::Map constructor usage everywhere

Adds NodeThreadPool implementation using AsyncQueue to call
Nan::AsyncQueueWorker from main thread

platform/default/thread_pool.cpp

@@ -0,0 +1,55 @@
+#include <mbgl/platform/default/thread_pool.hpp>
+#include <mbgl/actor/mailbox.hpp>
+
+namespace mbgl {
+
+ThreadPool::ThreadPool(std::size_t count) {
+    threads.reserve(count);
+    for (std::size_t i = 0; i < count; ++i) {
+        threads.emplace_back([this] () {
+            while (true) {
+                std::unique_lock<std::mutex> lock(mutex);
+
+                cv.wait(lock, [this] {
+                    return !queue.empty() || terminate;
+                });
+
+                if (terminate) {
+                    return;
+                }
+
+                auto mailbox = queue.front();
+                queue.pop();
+                lock.unlock();
+
+                if (auto locked = mailbox.lock()) {
+                    locked->receive();
+                }
+            }
+        });
+    }
+}
+
+ThreadPool::~ThreadPool() {
+    {
+        std::lock_guard<std::mutex> lock(mutex);
+        terminate = true;
+    }
+
+    cv.notify_all();
+
+    for (auto& thread : threads) {
+        thread.join();
+    }
+}
+
+void ThreadPool::schedule(std::weak_ptr<Mailbox> mailbox) {
+    {
+        std::lock_guard<std::mutex> lock(mutex);
+        queue.push(mailbox);
+    }
+
+    cv.notify_one();
+}
+
+} // namespace mbgl