Release v19.106.3

Summary

Groundwork for rebuilding a projection as a new generation while the current one keeps serving. Nothing uses it yet, and there is no behavior change. (part of #5158)

Added

• Kernel concepts for materialization generations: a generation number, its lifecycle states, and a fixed mapping from each generation's observer id to its container. Generation 1 keeps today's projection id and collection. (part of #5158)
• The kernel can resolve a sink for an explicit destination container, and build a projection pipeline keyed by generation. (part of #5158)
• A reserved ProjectionGenerations kernel feature setting, off by default and not used yet. (part of #5158)