Remove raw option from cas and add instrumentation

The `cas` and `cas_multi` operations incorrectly forced the `:raw` option. This changes those operations to respect the raw/encoded request.

Several methods were missing ActiveSupport::Notification instrumentation, so not all external calls could be tracked. This adds instrumentation to `cas`, `cas_multi`, `read_multi`, `clear` and `stats`.

@camilo @arthurnn for review.