Update seastar submodule
Update seastar submodule

* seastar 36cf5c5ff0...3f3e117de3 (16):
  > memcached: don't use C++17-only std::optional
  > reactor: Comment why _backend is assigned in constructor body
  > log: restore --log-to-stdout for backward compatibility
  > used_size.hh: Include missing headers
  > core: Move some code from reactor.cc to future.cc
  > future-util: move parallel_for_each to future-util.cc
  > task: stop wrapping tasks with unique_ptr
  > Merge "Setup timer signal handler in backend constructor" from Pavel
Fixes #5524
  > future: avoid a branch in future's move constructor if type is trivial
  > utils: Expose used_size
  > stream: Call get_future early
  > future-util: Move parallel_for_each_state code to a .cc
  > memcached: log exceptions
  > stream: Delete dead code
  > core: Turn pollable_fd into a simple proxy over pollable_fd_state.
  > Merge "log to std::cerr" from Benny

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