sharded: fix start with reference parameter
sharded: fix start with reference parameter

Currently reference cannot be passed to sharded::start(). It happens
because apply is called with rref, but expects to get lref (since Args&&
in apply's lambda is not really universal reference). Fix it.

Message-Id: <4fdc26e12c6bc3ac7e5f752376212d89dfc056c5@scylladb.com>

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