Instead of copying files in adapters, create hard links where possible (#2290)

Rebased #2120 to master.
Paperclip duplicates the original files quite a lot as part of its validation process. (#1642, #1326).
When uploading large files (several hundred megabytes to gigabyte range), this becomes a problem: The web server will be busy creating 3 - 4 duplicates on disk, while the app (and potentially the user) are waiting for the upload operation to complete.
This pull request introduces hard links instead of ```FileUtil.cp``` where possible to keep the logic as-is but save time and disk space.

lib/paperclip/io_adapters/abstract_adapter.rb

@@ -52,8 +52,18 @@ module Paperclip
     end
 
     def copy_to_tempfile(src)
-      FileUtils.cp(src.path, destination.path)
+      link_or_copy_file(src.path, destination.path)
       destination
     end
+
+    def link_or_copy_file(src, dest)
+      Paperclip.log("Trying to link #{src} to #{dest}")
+      FileUtils.ln(src, dest, force: true) # overwrite existing
+      @destination.close
+      @destination.open.binmode
+    rescue Errno::EXDEV, Errno::EPERM, Errno::ENOENT => e
+      Paperclip.log("Link failed with #{e.message}; copying link #{src} to #{dest}")
+      FileUtils.cp(src, dest)
+    end
   end
 end

lib/paperclip/io_adapters/attachment_adapter.rb

@@ -23,7 +23,7 @@ module Paperclip
 
     def copy_to_tempfile(source)
       if source.staged?
-        FileUtils.cp(source.staged_path(@style), destination.path)
+        link_or_copy_file(source.staged_path(@style), destination.path)
       else
         source.copy_to_local_file(@style, destination.path)
       end

lib/paperclip/storage/filesystem.rb

@@ -37,7 +37,7 @@ module Paperclip
         @queued_for_write.each do |style_name, file|
           FileUtils.mkdir_p(File.dirname(path(style_name)))
           begin
-            FileUtils.mv(file.path, path(style_name))
+            move_file(file.path, path(style_name))
           rescue SystemCallError
             File.open(path(style_name), "wb") do |new_file|
               while chunk = file.read(16 * 1024)
@@ -84,6 +84,17 @@ module Paperclip
       def copy_to_local_file(style, local_dest_path)
         FileUtils.cp(path(style), local_dest_path)
       end
+
+      private
+
+      def move_file(src, dest)
+        # Support hardlinked files
+        if File.identical?(src, dest)
+          File.unlink(src)
+        else
+          FileUtils.mv(src, dest)
+        end
+      end
     end
 
   end