seasetar: pick up the latest seastar

couple noticeable changes

* use dpdk v19.05, please note the older version of dpdk might not
  work, for instance dpdk v18.11 does not compile with the latest
  seastar master HEAD
* future cleanups

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

diff --git a/src/seastar b/src/seastar
index 0f1c5010063ecffb284804333e215fddd0cab9bc..7744693ca5696432ad90098846c753ae42af61e1 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 0f1c5010063ecffb284804333e215fddd0cab9bc
+Subproject commit 7744693ca5696432ad90098846c753ae42af61e1