when the second request did not happen within the default
backend_remote_error_holddown == 0.25s, we would see two ECONNREFUSED
instead of one and a holddown.

Seen in vtest:

**   top  === varnish v1 -expect VBE.vcl1.foo.fail_econnrefused > 0
**** dT   6.132
**   v1   as expected: VBE.vcl1.foo.fail_econnrefused (2) > 0
**   top  === varnish v1 -expect VBE.vcl1.foo.helddown > 0