future: clean-up the Result handling in then_wrapped().
future: clean-up the Result handling in then_wrapped().

Before the change the `Result` parameter was denoting the return
type of the entire `then_wrapped()` function. It was derivate of
return type of the `func` provided to `then_wrapped()`, obtained
by passing the type through `seastar::futurize`.

`FuncResult` denotes now the raw, non-futurized return type of
`func. Passing through `seastar::futurize` is deferred.

The commit incorporates changes from Rafael Avila de Espindola
<espindola@scylladb.com> posted on seastar-dev@googlegroups.com
during the review process (Msg ID: 87zhgqngoo.fsf@espindo.la).

Signed-off-by: Radoslaw Zarzynski <d31de3f997a9c1b9030b4b8b5b187b0d81e03886@redhat.com>
Message-Id: <1576260651-17205-1-git-send-email-d31de3f997a9c1b9030b4b8b5b187b0d81e03886@redhat.com>

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