Remove ORDER to make COUNT faster

[joelvh]

Lifted from graphql-ruby: https://github.com/rmosolgo/graphql-ruby/blob/master/lib/graphql/pagination/sequel_dataset_connection.rb#L19

[flash-gordon]

I genuinely wonder which DBMS doesn't optimize it itself...

[solnic]

@flash-gordon are you saying we should make this optimization on a per-db type basis?

[flash-gordon]

@solnic I'm saying that no DB doesn't do it by itself. Actually, using count with order by won't even work in PG, which means Sequel already ignores the ordering. This PR, however, reveals a bug in total because it uses unlimited datasets, which is plain wrong. If you want to paginate over a filtered relation, you'll get incorrect results. I'm amazed this hasn't been reported. I'll file an issue.

[joelvh]

@flash-gordon nice catch #366