add RemoteLogger for distributed logging

[simonbyrne]

Fixes JuliaLang/Distributed.jl#51

This is a pretty crude implementation: it simply remotecalls the logger on the main process, attaching a keyword pid to indicate which process triggered it, but I hope it can serve as a starting point.

The main downside of this approach is that it doesn't respect changes to the current logger on the main task, e.g.

with_logger(mylogger) do
   remotecall_wait(() -> @info("msg"), 2)
end

will still log to the global logger on the main process, not mylogger. This also means I can't test this using @test_logs. I don't see an easy way to fix this without something like #35757.

The other open question is when/where should the logger be instantiated?

[simonbyrne]

@c42f any thoughts?

[simonbyrne]

okay, added a test

[ViralBShah]

cc @tanmaykm

[simonbyrne]

bump?

[vtjnash]

I didn't review, but now needs to be reopened at https://github.com/JuliaLang/Distributed.jl for JuliaLang/Distributed.jl#52