seastar: pick up latest changes

* v2 api: remove variadic futures. let's be in sync with upstream.
* warn if the returned future is ignored. this helps us to catch
  bugs and to improve the readability of the code -- we need to
  explicitly ignore a future if we want to. and ideally, we should
  document the reason why a future is ignored.

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/seastar b/src/seastar
index 7744693ca5696432ad90098846c753ae42af61e1..f90da54293ff944abdb77b83af5f3d97cba4ea85 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 7744693ca5696432ad90098846c753ae42af61e1
+Subproject commit f90da54293ff944abdb77b83af5f3d97cba4ea85