Better match the behaviour of Advanced Post Cache to WP_Query when setting found_posts

[aidvu]

advance_post_cache was making the following choice in post_limits_request and posts_request:

• If the $limits part of the query is empty or no_found_rows is set return a count of 0.

We should split this, and return NO_FOUND_ROWS when no_found_rows is set, and return NO_LIMIT when there is no $limit clause, so the post count can be calculated correctly.

Also pulled in some code improvements.

[r-a-y]

Related: #21