Refactor DalliStore to better support subclassing

[jcxplorer]

As promised, here's a pull request that makes DalliStore a little easier to subclass, for example, to add support for a connection pool.

I took an approach that adds a #with_connection method through which the Dalli client is always accessed. It seemed like the best way to support a connection pool in a subclass, which can be done using the connection_pool gem:

module ActiveSupport
  module Cache
    class DalliPoolStore < DalliStore

      def initialize(pool, options={})
        @pool = pool
        @options = options
        extend Strategy::LocalCache
      end

      def with_connection(&block)
        @pool.with(&block)
      end

    end
  end
end

dalli_pool = ConnectionPool.new(size: 25, timeout: 5) { Dalli::Client.new("localhost:11211", threadsafe: false) }
config.cache_store = :dalli_pool_store, dalli_pool

Couple questions that popped up while I was doing this:

1. DalliStore#dalli is not used anywhere and the tests don't seem to mind if it's not defined. Was it used internally at some point or is it just to provide access to a Dalli::Client instance via Rails.cache.dalli? That method won't work with the example pool store above, but I think that's ok.
2. DalliStore#read_multi calls extract_options! on its arguments but doesn't assign its return value anywhere unlike other methods such as #fetch_multi. There's an options variable being used later which ends up being the options passed when the DalliStore is created. Is this intentional or a bug? I'm not familiar enough with the code to be able to tell :)

[mperham]

You don't need to call with_connection twice on these two lines. Reuse the connection.

[jcxplorer]

Here it is. Could do the same for read_entry and delete_entry but all places that call those don't have an open connection, so I'm not sure if it's worth the trouble.

I can squash those commits once you think it's good to go.

[mperham]

Good enough, squash away!

[jcxplorer]

All done, thanks a lot for reviewing this!

[mperham]

I've added connection pool support into the standard dalli_store adapter. See c51ce5d