An impossibly large refactor

[winmillwill]

Includes the build-docker-images branch.

The commits are all distinct and understandable except the last one which is:

1. introducing typedefs for sdata and pdata to shrink boilerplate
2. consistently using the const funcname pattern for log messages instead of pasting the name as a string literal everywhere
3. refactoring filecache_sync to favor early returns over else and goto
4. refactoring get_fresh_fd to

• dispatch separate http outcomes to distinct handler functions
• use newly created reusable _filecache_truncate helper, factored out of filecache_truncate
• offload cache freshness determination to a helper function
• similarly favor early return over else and goto
• fit on a screen

[pantheon-circleci]

f28-1505-devel-501 https://circleci.com/workflow-run/ec3dd06d-639f-4e46-90b8-44cd655201c0

[pantheon-circleci]

f28-1509-devel-501 https://circleci.com/workflow-run/f7359548-52dd-4a4c-adb5-fadfb9eac8d9