loading `DateTime` arrays is very slow

[JeffBezanson]

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Compared against deserialize. For example

dts = [ DateTime()+Dates.Millisecond(i) for i=1:1000000 ]
JLD.save("test.jld", "x", dts);
open(f->serialize(f,dts),"test.jls","w")

# after warm-up
julia> @time JLD.load("test.jld");
  0.073771 seconds (2.00 M allocations: 45.800 MB, 9.72% gc time)

julia> @time open(deserialize,"test.jls");
  0.005927 seconds (36 allocations: 7.632 MB)

JLD is more than 10x slower in this case.