mutation_reader: extract the two fill_buffer_result into a single one
mutation_reader: extract the two fill_buffer_result into a single one

Currently we have two, nearly identical definitions of said struct.
Extract it to a common definition and rename it to
`remote_fill_buffer_result`.

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