Implement Collection.putbatch()

[dw]

Some method that allows bulk loading of batch records, without first saving individual records to the store, just to later delete them during batch().

Using existing API pattern, putbatch() call would be pretty ugly: either it would be putbatch(recs, keys=None), which is horribly unintuitive, or would involve some putbatchitems(pairs), which looks just as bad.

So perhaps re-think Collection API before implementing this

[dw]

Providing this interacts annoyingly with #70, without tightly coupling Strategy & Collection, or providing a Collection.putbatch() that raises an exception unless Strategy.putbatch() is implemented.