Fix JET errors around matching methods for send_msg_now(...)

[DilumAluthge]

This is a forward-port of JuliaLang/Distributed.jl#165 (JuliaLang/Distributed.jl@871e3d7).

┌  @ Distributed /workpath/Distributed.jl/src/process_messages.jl:321
│ no matching method found `send_msg_now(::Distributed.LocalProcess, ::Distributed.MsgHeader, ::Distributed.IdentifySocketAckMsg)` (1/2 union split): Distributed.send_msg_now(w, Distributed.MsgHeader()::Distributed.MsgHeader, Distributed.IdentifySocketAckMsg()::Distributed.IdentifySocketAckMsg)
└────────────────────

┌  @ Distributed /workpath/Distributed.jl/src/process_messages.jl:359
│ no matching method found `send_msg_now(::Distributed.LocalProcess, ::Distributed.MsgHeader, ::Distributed.JoinCompleteMsg)` (1/2 union split): Distributed.send_msg_now(controller::Union{Distributed.LocalProcess, Distributed.Worker}, Distributed.MsgHeader(Distributed.RRID(0, 0)::Distributed.RRID, (header::Distributed.MsgHeader).notify_oid::Distributed.RRID)::Distributed.MsgHeader, Distributed.JoinCompleteMsg((Distributed.Sys).CPU_THREADS::Int64, Distributed.getpid()::Int32)::Distributed.JoinCompleteMsg)
└────────────────────

┌ connect_to_peer(manager::Distributed.ClusterManager, rpid::Int64, wconfig::Distributed.WorkerConfig) @ Distributed /workpath/Distributed.jl/src/process_messages.jl:368
│ no matching method found `send_msg_now(::Distributed.LocalProcess, ::Distributed.MsgHeader, ::Distributed.IdentifySocketMsg)` (1/2 union split): Distributed.send_msg_now(w, Distributed.MsgHeader()::Distributed.MsgHeader, Distributed.IdentifySocketMsg(myid()::Int64)::Distributed.IdentifySocketMsg)
└────────────────────

┌ send_msg_now(s::Base.PipeEndpoint, header::Distributed.MsgHeader, msg::Distributed.ResultMsg) @ Distributed //workl/clones/JuliaLang/Distributed.jl/src/messages.jl:113
│ no matching method found `send_msg_now(::Distributed.LocalProcess, ::Distributed.MsgHeader, ::Distributed.(1/2 union split): Distributed.send_msg_now(Distributed.worker_from_id(id::Int64)::Union{Distributed.LocalProcess, Worker}, header::Distributed.MsgHeader, msg::Distributed.ResultMsg)
└────────────────────

(cherry picked from commit 871e3d746ab0c7668462f7b88515a362da9c11f1)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.39%. Comparing base (fbca46f) to head (a44c864).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   87.39%   87.39%           
=======================================
  Files          11       11           
  Lines        2086     2086           
=======================================
  Hits         1823     1823           
  Misses        263      263           

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