improve various inferrabilities, fix some errors (JuliaLang/julia#40463)

Author: aviatesk

src/cluster.jl

@@ -171,7 +171,7 @@ function check_worker_state(w::Worker)
     end
 end
 
-exec_conn_func(id::Int) = exec_conn_func(worker_from_id(id))
+exec_conn_func(id::Int) = exec_conn_func(worker_from_id(id)::Worker)
 function exec_conn_func(w::Worker)
     try
         f = notnothing(w.conn_func)

src/remotecall.jl

@@ -259,7 +259,7 @@ function send_del_client(rr)
     if rr.where == myid()
         del_client(rr)
     elseif id_in_procs(rr.where) # process only if a valid worker
-        w = worker_from_id(rr.where)
+        w = worker_from_id(rr.where)::Worker
         push!(w.del_msgs, (remoteref_id(rr), myid()))
         w.gcflag = true
         notify(any_gc_flag)