Investigate minibatch LBFGS

[accosmin]

• see https://arxiv.org/pdf/1605.06049v2.pdf
• also this as it may be related: https://arxiv.org/pdf/1609.04836.pdf
• need to extend iterator_t to have an overlap between consecutive batches

[accosmin]

• should also implement a minibatch trainer: run a batch solver for couple of iterations with the SAME minibatch (parameters: minibatch = 32-1024, iters = 4-32)