dnscache: slight refactoring

dnscache: slight refactoring

Slight refactoring around dnscache, e.g. hostcache

- eliminate `data->state.hostcache`. Always look up
  relevant dnscache at share/multi.
- unify naming to "dnscache", replacing "hostcache"
- use `struct Curl_dnscache`, even though it just
  contains a `Curl_hash` for now.
- add `Curl_dnscache_destroy()` for cleanup in
  share/multi.

Closes #16941