PERF: Split queries when cleaning uploads.

This reduces the number of scans that the db has to do in the query
to fetch orphan uploads. Futheremore, we were not batching our
records which bloats memory.

spec/fabricators/upload_fabricator.rb

@@ -1,11 +1,11 @@
 Fabricator(:upload) do
   user
-  sha1 "e9d71f5ee7c92d6dc9e92ffdad17b8bd49418f98"
+  sha1 { sequence(:sha1) { |n| Digest::SHA1.hexdigest(n.to_s) } }
   original_filename "logo.png"
   filesize 1234
   width 100
   height 200
-  url  "/uploads/default/1/1234567890123456.png"
+  url { sequence(:url) { |n| "/uploads/default/#{n}/1234567890123456.png" } }
 end
 
 Fabricator(:attachment, from: :upload) do

spec/fabricators/user_avatar_fabricator.rb

@@ -0,0 +1,3 @@
+Fabricator(:user_avatar) do
+  user
+end