The assertions already have produces(mutation) and
produces(dht::decorated_key) overloads. Additional overload that accepts
a range of elements will allow to check if a range of mutations of
decorated keys is produced.
The same interface is exposed by mutation_reader_assertions.
produces(mutation_fragment::kind) is provided by
streamed_mutation_assertions and is going to be needed in order to
fully convert tests to the flat mutation readers.
Both fast_forward_to() overloads return a future which should be waited
for. Additionally, fast_forward_to(const dht::partition_range&) expects
the range to remain valid at least until the next call to
fast_forward_to(). The original mutation_reader_assertions guaranteed
that and so should flat_mutation_reader_assertions.
