Avoid a duplicate query for the comment count

[rmccue]

Back in 768a282 (#1197), we introduced the regular pagination headers for comment queries. However, apparently we missed that WP_Comment_Query has SQL_CALC_FOUND_ROWS built-in (but off by default), so we're doing a complex query twice here.

Attached patch uses $query->found_comments and $query->max_num_pages instead of redoing the query.

[rmccue]

This was introduced originally in #1209, since (as the failing test shows) out-of-bounds pages will give back 0 for SQL_CALC_FOUND_ROWS. Unfortunately, this hurts common-case performance (in-bounds requests) to support an edge case (out-of-bounds requests).

Changed this to using the normal query on in-bounds requests, and only running the secondary query if we're out of bounds. Updating other controllers now.

[rmccue]

@WP-API/amigos #reviewmerge

[danielbachhuber]

Looks good 👍 And well-covered by existing tests.

[danielbachhuber]

Related:

• https://core.trac.wordpress.org/ticket/29178#comment:3
• https://core.trac.wordpress.org/changeset/34544

Although, in this case, I think this PR is still better than what we had before.