Actually clear all DNS stats on resolver changes

Previously, we were clearing the stats for the first resolver 4 times
in a loop :-)

Also:
 - Factor revision_id bumping into res_cache_clear_stats_locked()
   because it must be done every time.
 - Delete redundant stats clearing in free_nameservers_locked()

Test: atest (but tests don't actually detect this bug)
Change-Id: I770a99fd6666bb269bc4c7355e06ca24095ed6ee