Using a lambda for implementing a mutation_reader is nifty, but does not
allow us to add methods.
Switch to a class-based implementation in anticipation of adding a close()
method.
make_reader_returning() is used by the single-key query path, and is slowed
down by needlessly allocating a vector, which is initialized by copying
the mutation (as initializer_list<> can not be moved from).
Fix by giving it its own implementation instead of relying on
make_reader_returning_many().
Many mutation_reader implementations capture 'this', which, if copied,
becomes invalid. Protect against this error my making mutation_reader
a non-copyable object.
Fix inadvertant copied around the code base.
