Having collection caching that wraps templates and automatically tries
to infer if they are cachable proved to be too much of a hassle.
We'd rather have it be something you explicitly turn on.
This removes much of the code and docs to explain the previous automatic
behavior.
This change also removes scoped cache keys and passing cache_options.
Collections can take advantage of `multi_read` if they render one template
and their partials begin with a cache call.
The cache call must correspond to either what the collections elements are
rendered as, or match the inferred name of the partial.
So with a notifications/_notification.html.erb template like:
```ruby
<% cache notification %>
<%# ... %>
<% end %>
```
A collection would be able to use `multi_read` if rendered like:
```ruby
<%= render @notifications %>
<%= render partial: 'notifications/notification', collection: @notifications, as: :notification %>
```
