mutation_reader: adjust FragmentProducer concept for timeout
mutation_reader: adjust FragmentProducer concept for timeout

forward_to() no accepts a timeout parameter, and the concept should
reflect it, or it breaks the build when concepts are enabled.

avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla