Update seastar submodule
Update seastar submodule

* seastar 2b510220...cdda3051 (10):
  > core: discard unused variable / function
  > pollable_fd: use boost::intrusive_ptr rather than std::unique_ptr for lifecycle management
  > build: check for pthread_setname_np()
  > build: link against Threads::Threads
  > future: Avoid recursion in do_for_each
  > future: Expand description of parallel_for_each
  > merge: Add content length limit to httpd
  > tests/scheduling_group_test: verify current scheduling group is inherited as expected
  > net: return future<> instead of subscription<>
  > cmake: be more verbose when looking for libraries

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