future: pass func as mutable to lambda arg of handle_exception[_type]
future: pass func as mutable to lambda arg of handle_exception[_type]

When func is const, as it is now, we cannot use mutable closure in
lambda, for example construct like this:

  future<> f(sometype &t) {
    return run_smth().handle_exception([&t] (std::exception) mutable {
      t.handle_failure();
      std::cerr << "Failed" << std::endl;
    });
  }

fails with error about dropping "const" qualifier from "this" when calling
lambda.

Message-Id: <f0147090ee7c8dd80cff1a976568d0947c319589@gmail.com>

avikivity/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar