Cache reflection plan for mapping services

[JeremyCaney]

Instead of re-deriving the same reflection facts on every single topic mapping request, cache them once.

Tasks

• Cache constructor, collection, factory reflection lookups instead of resolving them per topic; on TopicMappingService:
  • Activator.CreateInstance() call
  • InitializeCollection()
  • TopicFactory.Create()
• Cache other type-specific facts recomputed per topic on TypeAccessor and MemberAccessor:
  • Member lists
  • Validator lists
  • Composite attribute keys
• Restrict reflection to public members only, matching what can actually be set (TypeAccessor, MemberAccessor)
  • This fixes a potential crash when trying to set a private setter
• Build a single compiled "mapping plan" per view-model type, subsuming per-page re-derivation (TopicMappingService); see Preload TopicPayload as part of MapAsync() #133