Prevent scavenger from removing build ids that were recently default for a set

[bergundy]

Part 3 of the last minute refactoring of versioning data timestamps (see #4524 and #4526).

With this change, the scavenger uses the build id became_default_timestamp to determine whether it should be considered for removal.

From the documentation in code:

// If a build id was still default recently, there may be:
// 1. workers with that identifier processing tasks
// 2. workflows with that identifier that have yet to be indexed in visibility
// The scavenger should allow enough time to pass before cleaning these build ids.

Note that this is stacked on top of #4526 and only the last commit should be reviewed.