Update seastar submodule
Update seastar submodule

* seastar 3c60b82...7aca670 (2):
  > Merge "Log stack trace during exception" from Gleb
  > shared_ptr: Introduce lw_shared_ptr::dispose() for convenience

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