caddy-frontend: Keep connection to the backend

There is no reason to drop connection to the backend and then reinitate it on
each request. It's better to keep connected and avoid additional work,
especially in SSL handshake.

Note: This was kept as the file has been copied from other place (haproxy
      on a backend), and slipped MR review, where MR author wanted to drop it
      anyway.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:29:03

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:09:54

passed SlapOS.SlapRunner.ResilienceTest-Master

00:28:00