input_stream: un-unroll input_stream::consume()
input_stream: un-unroll input_stream::consume()

input_stream::consume has two different paths depending
on whether the consumer returned a ready or non-ready future,
but these paths are redundant as future::then() also has these
two paths. It also has a TODO for deferring when too many loops
were executed, but the outer repeat() already does that.

Consolidate the two paths into one.
Message-Id: <a7fae8ed4de3972bea8b738ba9894786f52fc6cf@scylladb.com>

scylladb/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar