lwt: do not copy proposal in paxos_state::accept
lwt: do not copy proposal in paxos_state::accept

A proposal is passed as a reference and all callers have it in stable
memory until the call ends, so it is safe to use the reference
everywhere.

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