Use std::optional instead of mbgl::optional (#911)

src/mbgl/tile/raster_tile.cpp

@@ -35,7 +35,7 @@ void RasterTile::setError(std::exception_ptr err) {
     observer->onTileError(*this, std::move(err));
 }
 
-void RasterTile::setMetadata(optional<Timestamp> modified_, optional<Timestamp> expires_) {
+void RasterTile::setMetadata(std::optional<Timestamp> modified_, std::optional<Timestamp> expires_) {
     modified = std::move(modified_);
     expires = std::move(expires_);
 }