perf(wip): optimize count=exact when there's no limits, offsets or db-max-rows

[laurenceisla]

Closes #3957

[steve-chavez]

@laurenceisla Could you inline all the context here? 🙏 I'm finding difficult to understand the PR.

[laurenceisla]

@steve-chavez Yes, so the requests handle two counts when using the Prefer: count=exact header in the request. This is to build the two parts of the Content-Range header. For example, let's take our items table:

$ curl 'localhost:3000/items?limit=5' -H 'Prefer: count=exact' -I
HTTP/1.1 206 Partial Content
...
Content-Range: 0-4/15

To build the Content-Range, It counts the limited query to get the 0-4 and the total query to get the 15 (two counts in total).

However if there's nothing that would limit the request (limit, offset or db-max-rows) then we'd get:

$ curl 'localhost:3000/items' -H 'Prefer: count=exact' -I
HTTP/1.1 200 OK
...
Content-Range: 0-14/15

The total quantity would always be the same for both parts of the Content-Range: 0-14 and 15 (15 items for both). So, this PR only counts once (the 0-14 part) and uses the same result to build the second part of the Content-Range (the 15 part).