splay: use access functions, add asserts, use Curl_timediff

splay: use access functions, add asserts, use Curl_timediff

- add set/get functions for the custom data in a tree node

- use Curl_timediff for time comparisons instead of the custom macro, as they
  do the exact same things.

- add asserts to catch mistakes better

- updated test 1309 accordingly

Closes #14562