file_data_source_impl: Make issue_read_aheads() exception-safe
file_data_source_impl: Make issue_read_aheads() exception-safe

_read_buffers.reserve() ensures that later push() doesn't throw and
thus doesn't abandon the reading fiber.

The reading fiber is wrapped with futurize::apply() so that any
exceptions are propagated into then_wrapped() and not thrown directly,
which would for example corrupt _reads_in_progress.

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