NEXUS-5511: Improve INFEx reasoning.

Introducing new RequestProcessor2 iface to completely replace
existing RequestProcessor's. New iface has means to provide
"proper reasoning" why something should not be found.

Also, unlike old iface, new one is able to interfere with INFex
only, not with other (usually problem meaning) exceptions
like IllegalOperationEx or AccessDeniedEx.

An adapter is provided to wrap old implementations if needed,
but cleanest way for RequestProcessor implementors would be to
reimplement their code agains new iface.