future: expose more types in continuation_base
future: expose more types in continuation_base

Make it easy for template users to find out what the future
and promise types are for this continuation.
Message-Id: <22ac53a463a702e0685411c16cb353948c045b90@scylladb.com>

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