tls: fix tls-alpn-01 flow

[mrl5]

pingoo exits before tls-alpn-01 flow finishes

this is a regression introduced in commit 4d2c340 -- release v0.14.9

steps to reproduce

0. (probably optional) use standalone binary
1. setup DNS record e.g. foo.bar
2. make sure /etc/pingoo/tls is empty
3. set minimal tls config - e.g.:

listeners:
  https:
    address: https://0.0.0.0

tls:
  acme:
    domains: ["foo.bar"]

services:
  api:
    http_proxy: ["http://127.0.0.1:3000"]

4. run some example service on port 3000 -- e.g. pong.rs
5. run pingoo - e.g. with sudo ./pingoo

expected result

1. TLS certificate is obtained from Lets Encrypt and saved to /etc/pingoo/tls/
2. pingoo accepts https requests

actual result

pingoo exits with 0 before certificates are written