Resampling strategies should have option for independent RNG

[ablaom]

Currently the global RNG is used in Holdout and CV, which is bad for reproducibility. I suggest we add an extra field to these strategies, rng which is an integer seed for a MersenneTwister (global RNG seed) or an arbitrary AbstractRNG object, as is done, for example, in DeterministicEnsembleModel. From that code:

mutable struct DeterministicEnsembleModel{Atom<:Deterministic} <: Deterministic
    atom::Atom
    weights::Vector{Float64}
    bagging_fraction::Float64
    rng::Union{Int,AbstractRNG}    # <----------
    n::Int
    parallel::Bool
    out_of_bag_measure # TODO: type this
end

And later in the fit method:

 ....
    if model.rng isa Integer
        rng = MersenneTwister(model.rng)
    else
        rng = model.rng
    end
... <code using rng to generate random numbers>