BURs: update posts in parallel.

When processing an alias, rename, implication, mass update, or nuke,
update the posts in parallel. This means that if we alias foo to bar,
for example, then we use four processes at once to retag the posts from
foo to bar.

This doesn't mean that if we have two aliases in a BUR, we process both
aliases in parallel. It simply means that when processing an alias, we
update the posts in parallel for that alias.

Gemfile

@@ -56,6 +56,7 @@ gem 'clockwork'
 gem 'puma-metrics'
 gem 'puma_worker_killer'
 gem "rack-timeout", require: "rack/timeout/base"
+gem "parallel"
 
 group :production do
   gem 'unicorn', :platforms => :ruby