Feature freeze adjustment to manage dependencies better

By splitting up the FeatureFreeze from being monolithic into classes of packages, we can manage the risk of incorporating changes into the release, and maximize the development time for bulk of the distribution.

Areas of increasing risk:
- "ToolchainFreeze" - for gnu toolchain - ABI changes in particular.
- "PlumbingFreeze" - for packages that have significant dependency chains, and need time to stabilize. List of packages to be defined by FeatureDefinitionFreeze, possibly use "core" seed as guidance?
- "FeatureFreeze" - for rest of packages (traditional definition).