fstream: do not ignore unresolved future
fstream: do not ignore unresolved future

Even after _done is completed some of the read buffers may have still
unresolved futures if there was a skip that has attached additional
continuations.

Fixes #322.

Message-Id: <1fa78b7a66bbcdcbeb0eb3f5106cd126bf696700@scylladb.com>

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