table.where reverse iterator

[exrich]

Rationale:

I had a problem with a table.where (indexed) query returning a lot of results but I only wanted the last row of data for that query. For example if your table represented a timeseries and you only wanted the most recent result before a certain date. Currently table.where would read in the chunks from first to last chunk so you would have to read in the entire iterator to get to your last row. This is slow since you are making a lot of table._read_chunk calls.

With this reverse iterator you can specify a step of -1 and just read in the first value of the iterator resulting in a large speedup.

Changes pass the light test suite. Couldn't get the heavy to run. Are they broken?

functionality improved to take a step of -1 so that you can read the iterator in reverse order (indexed queries only so far)