Right now the DDL operations does not respect running index builds.
This can lead to availability issues like in HELP-97655 and HELP-97212.
Some custom error handling is already applied to handle this edge case in some DDL coordinator (like SERVER-117624), but there is no generic solution implemented yet.
This task is about adding a generic best effort check for DDL operations to avoid running during an active index build on the affected namespaces.

• The solution should be a best effort check prevent starting of DDL operations if there is an index build
  • We accept that some cases will slip through the check
• The index builds should not respect the running DDL operations
• The DDL operations should be able to exclude themselves from this check
  • By reporting the affected namespaces - exclusion is by reporting an empty set
  • By default the affected namespaces are the locked ones
• collmod should keep its own way of the index build handling, this solution does not replace it just comes on to