NEXUS-5511: Simplification and method renames

Simplified RequestProcessor2 by removing a not used
call shouldCache. This would not even work properly
as would circumvent retries and other logic from core

Methods on RequestProcessor2 renamed to better reflect
when they are called.
NEXUS-5511: Revamped request processor API

Backed off from initial design, and returned to throwing an
exception when some signaling is needed from processor.