TUN-3902: Add jitter to backoffhandler

Jitter is important to avoid every cloudflared in the world trying to
reconnect at t=1, 2, 4, etc. That could overwhelm the backend. But
if each cloudflared randomly waits for up to 2, then up to 4, then up
to 8 etc, then the retries get spread out evenly across time.

On average, wait times should be the same (e.g. instead of waiting for
exactly 1 second, cloudflared will wait betweeen 0 and 2 seconds).

This is the "Full Jitter" algorithm from https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/

cfsetup.yaml

@@ -194,6 +194,7 @@ stretch: &stretch
     builddeps:
       - *pinned_go_fips
       - build-essential
+      - gotest-to-teamcity
     post-cache:
       - export GOOS=linux
       - export GOARCH=amd64
@@ -201,7 +202,7 @@ stretch: &stretch
       # cd to a non-module directory: https://github.com/golang/go/issues/24250
       - (cd / && go get github.com/BurntSushi/go-sumtype)
       - export PATH="$HOME/go/bin:$PATH"
-      - make test
+      - make test | gotest-to-teamcity
   update-homebrew:
     builddeps:
       - openssh-client