Use local cache Rack middleware with BulkData::Cache

This sets up a local cache that wraps around the Redis cache layer of
BulkData::Cache during the duration of a request.

In order for this to work in a development environment we need the
BulkData::Cache class not to be reloaded automatically. I achieved this
by doing direct require calls. I first tried using the Rails
autoload_once_paths option but didn't have much luck.

As an example, before fixed require calls:

```
irb(main):002:0> BulkData::Cache.object_id
=> 70342543814920
irb(main):003:0> reload!
Reloading...
=> true
irb(main):004:0> BulkData::Cache.object_id
=> 70342595490220
```

and after:

```
irb(main):002:0> BulkData::Cache.object_id
=> 70249028613320
irb(main):004:0> reload!
Reloading...
=> true
irb(main):005:0> BulkData::Cache.object_id
=> 70249028613320
```

config/initializers/bulk_data_rack_middleware.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require "bulk_data/cache"
+
+# This is used to set a local cache that runs on BulkData for the duration of
+# a request. This means if we look up the same item in the cache multiple
+# times during a single request it will only hit Redis once and then look up
+# the item in memory.
+#
+# For more details see: https://github.com/rails/rails/blob/fa292703e1b733a7a55a8d6f0d749ddf590c61fd/activesupport/lib/active_support/cache/strategy/local_cache.rb
+Rails.application.config.middleware.insert_before(
+  ::Rack::Runtime,
+  BulkData::Cache.middleware,
+)

lib/bulk_data/cache.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "bulk_data"
+
 module BulkData
   class Cache
     include Singleton
@@ -8,6 +10,7 @@ class NoEntryError < RuntimeError; end
 
     class << self
       delegate :cache, to: :instance
+      delegate :clear, :middleware, to: :cache
     end
 
     attr_reader :cache