checksum_timeout - Use consistent naming

checksum_timeout - Use consistent naming

This was a weird one:

 * The admin form rendered HTML element "checksumTimeout" but saved setting "checksum_timeout", which required workarounds.
 * The runtime code read the setting as "checksum_timeout".
 * The metadata defined setting "checksumTimeout". `prefetch` is enabled, but there's no `config_key` or `legacy_key`.

I believe this means that the runtime and admin form worked (because they
agreed that the name was "checksum_timeout"), but the Setting API would not
work (because it used "checksumTimeout").

This revision standardizes on "checksum_timeout" on the theory that
"checksum_timeout" was the actual value used at runtime. This means:

 * If you had configured the form and had it working correctly, it should
   still work correctly.
 * If you had a script to set "checksumTimeout" via API, then it wasn't
   working before, and it's still not working now.