make it easy to bring up another instance for verifying that a new index is served correctly

[stapelberg]

This includes:

• adding a -listen_host option to source-backend
• possibly unifying index-backend’s -listen to -listen_host
• creating a little script which generates a new canary config by replacing the port numbers from the live config so that any changes are always taken over without having to care
• check from the script whether the new site serves (also check before that it doesn’t serve)
• check the index size and see if it differst more than x%
• if everything is okay, swap the indexes
• in dcsindex and dcsunpack, switch from log to printf to get the output on stdout instead of stderr

This enables automatic updates from crontab :).

[stapelberg]

See also comment number 3 in #1, which would solve this issue trivially. One would just need to bring up a batch stack.

[stapelberg]

commit dd72846 solves most of the issues here.

What’s still missing is update.sh itself, plus the check for the index size difference.

[stapelberg]

I skipped the index size difference check for now, but the rest is implemented. Let’s see how well it does in practice next friday :).