Add an ID to Environment

Environment will now have an incremental unique ID to that will
help distinguish between threads scoped to different Environments
when logging.

src/mbgl/map/environment.cpp

@@ -3,6 +3,7 @@
 
 #include <uv.h>
 
+#include <atomic>
 #include <cassert>
 #include <mutex>
 #include <unordered_map>
@@ -66,6 +67,11 @@ private:
     mutable std::mutex mtx;
 };
 
+unsigned makeEnvironmentID() {
+    static std::atomic<unsigned> id(0);
+    return id++;
+}
+
 ThreadInfoStore threadInfoStore;
 
 } // namespace
@@ -80,7 +86,8 @@ Environment::Scope::~Scope() {
     threadInfoStore.unregisterThread();
 }
 
-Environment::Environment(FileSource& fs) : fileSource(fs), loop(uv_loop_new()) {
+Environment::Environment(FileSource& fs)
+    : id(makeEnvironmentID()), fileSource(fs), loop(uv_loop_new()) {
 }
 
 Environment& Environment::Get() {
@@ -102,6 +109,10 @@ std::string Environment::threadName() {
     return threadInfoStore.getThreadInfo().name;
 }
 
+unsigned Environment::getID() const {
+    return id;
+}
+
 void Environment::requestAsync(const Resource& resource,
                                std::function<void(const Response&)> callback) {
     fileSource.request(resource, *this, std::move(callback));

src/mbgl/map/environment.hpp

@@ -41,6 +41,7 @@ public:
     static bool currentlyOn(ThreadType);
     static std::string threadName();
 
+    unsigned getID() const;
     void requestAsync(const Resource&, std::function<void(const Response&)>);
     Request* request(const Resource&, std::function<void(const Response&)>);
     void cancelRequest(Request*);
@@ -49,6 +50,7 @@ public:
     void terminate();
 
 private:
+    unsigned id;
     FileSource& fileSource;
 
 public: