add bulk_walk method for walking using get_bulk instead of get_next

This is a first draft for how bulk walking could work.

I used as much as i could from the walk method, but because get_bulk requests return multiple data rows, i had to "validate" rows in a different way.

i have to split data into columns, and push data as the come in, and then creating varbind_lists from the first value in each column if the index matches the index_column varbind.

It is a bit weird, but if i do it the way walk does, and get columns from different tables where not all indexes exists in one table, then the data would get out of sync.

Anyway, it is hard to explain, but take a look at the pull request and tell me what you think.