Work around memory leak: exporting a big database table consumes A LOT memory

Problem/Motivation

I tried to export a really big client db with --split-destination option this tool provides, but still I had to set PHP's memory limit to 8G.

Steps to reproduce

With 1G memory limit, try to export a db which has a table with about 400 000 records.

Proposed resolution

Export big tables into multiple files.

API changes

New --subsplit-limit option.