Optimize count_all_messages()

When counting the exact number of messages, `dm.Message.grep()[0]` is used to
get the total. Since no arguments are given, no filters are added to the
`Message.query`. However, with the default `defer=True` argument, the grep method
returns the total count, the number of pages, and `Message.query.all()`.
Thankfully, we can pass `defer=True` and it won't call `all()` on the query.

:warning: untested :construction: