Default Selectors are not unique

[mhauru]

using DynamicPPL

function make_selectors()
    s1 = DynamicPPL.Selector()
    s2 = DynamicPPL.Selector()
    return s1, s2
end

for i in 1:10
    s1, s2 = make_selectors()
    println("$(s1.gid == s2.gid)")
end

Indeterministic example output:

true
true
true
false
false
true
true
false
true
true

This is causing indeterministic test failures over at TuringLang/Turing.jl#2328

The cause is this:

function Selector(tag::Symbol=:default, rerun=tag != :default)
    return Selector(time_ns(), tag, rerun)
end

Sometimes to calls execute within a nanosecond, sometimes they don't.