Merge branch 'evdns-fail-requests-v6'

This patchset fixes some issues wit evdns_base_free(..., fail_requests=1), and
there are two cases: with callback wrapper for evdns_getaddrinfo() and not,
both fixed by this patches, and adds regression tests for this.

* evdns-fail-requests-v6:
  evdns: evdns_base_free(): fix UAF of evdns_base with @fail_requests
  test/dns: cover evdns_getaddrinfo() and evdns_base_free() with @fail_requests
  evdns: evdns_base_free(): free requests before namservers
  test/dns: cover @fail_requests for evdns_base_free()
  test/dns: more graceful coverage of @fail_requests