db,view: add a helper result builder class
db,view: add a helper result builder class

In order to migrate from mutation-based restriction checks,
code in view.cc needs to have a way of translating results
to partition-slice-based representation.
A slightly simplified builder from multishard_mutation_query.cc
is injected into the view code.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla