seastar: pick up changes for better performance

to be specific, a78fb44c96e2912c6f39b2151f94a0bb2b5796a6 helps to
improve the performance of future implementation -- with this change
future can always reference its local state without checking its `_promise`
and dereferencing it.

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

diff --git a/src/seastar b/src/seastar
index 23c9610a820dc2b85fcb97f0579335960f16cb4d..0f1c5010063ecffb284804333e215fddd0cab9bc 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 23c9610a820dc2b85fcb97f0579335960f16cb4d
+Subproject commit 0f1c5010063ecffb284804333e215fddd0cab9bc