Update seastar submodule
Update seastar submodule

* seastar a44cedf...d59fcef (10):
  > dns: Set tcp output stream buffer size to zero explicitly
  > tests: add libc-ares to travis dependencies
  > tests: add dns_test to test suite
  > build: drop bundled c-ares package
  > prometheus: replace the instance label with an optional one
  > build: Refactor C++ dialect detection
  > build: add libatomic to install-depenencies.sh
  > core: use std::underlying_type for open_flags
  > core: introduce open_flags::operator&
  > core: Fix build for `gnu++14`

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla