Use caddy's certmagic library for extensible/robust ACME handling (#14177)

* use certmagic for more extensible/robust ACME cert handling

* accept TOS based on config option

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/go.uber.org/atomic/.travis.yml

@@ -0,0 +1,27 @@
+sudo: false
+language: go
+go_import_path: go.uber.org/atomic
+
+env:
+  global:
+    - GO111MODULE=on
+
+matrix:
+  include:
+  - go: 1.12.x
+  - go: 1.13.x
+    env: LINT=1
+
+cache:
+  directories:
+    - vendor
+
+before_install:
+  - go version
+
+script:
+  - test -z "$LINT" || make lint
+  - make cover
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)