Fix some JET errors around matching methods for send_connection_hdr(...) and send_msg_now(...) (#180)

```
┌ create_worker(manager::Distributed.ClusterManager, wconfig::Distributed.WorkerConfig) @ Distributed ~/Distributed.jl/src/cluster.jl:709
│ no matching method found `send_connection_hdr(::Distributed.LocalProcess, ::Bool)` (1/2 union split): Distributed.send_connection_hdr(w::Union{Distributed.LocalProcess, Distributed.Worker}, true)
└────────────────────
```

```
┌ create_worker(manager::Distributed.ClusterManager, wconfig::Distributed.WorkerConfig) @ Distributed ~/Distributed.jl/src/cluster.jl:712
│ no matching method found `send_msg_now(::Distributed.LocalProcess, ::Distributed.MsgHeader, ::Distributed.JoinPGRPMsg)` (1/2 union split): Distributed.send_msg_now(w::Union{Distributed.LocalProcess, Distributed.Worker}, Distributed.MsgHeader(Distributed.RRID(0, 0)::Distributed.RRID, ntfy_oid::Distributed.RRID)::Distributed.MsgHeader, join_message)
└────────────────────
```

(cherry picked from commit 3e74cbc)

Author: DilumAluthge

src/cluster.jl

@@ -626,7 +626,7 @@ function create_worker(manager::ClusterManager, wconfig::WorkerConfig)
         end
     end
 
-    w = Worker(w_stub.id, r_s, w_s, manager; config=wconfig)
+    w = Worker(w_stub.id, r_s, w_s, manager; config=wconfig)::Worker
     # install a finalizer to perform cleanup if necessary
     finalizer(w) do w
         if myid() == 1