Possible to use more connections for the "default cache"

[walro]

Right now, an instance of Twingly::UrlCache can only hold a single connection to the backing cache. In some cases this will be a bottle-neck, in particular when you have high latency to the backing storage.

Just the gist of how it could work:

Twingly::UrlCache.new(connections: 10)

# internally we'd setup 10 connections
# easiest way is probably just 10 instances of the `Dalli` client from where we pick one at random