Steadily increasing memory consumption around RxRewriter #322

Main cause of openzim/warc2zim#383 seems to be around RxRewriter class and more specifically the usage of self in the replace function:

• when I do not use self in the replace function, warc2zim memory usage is very stable and low
• as soon as I use self in the replace function, even for a very silly thing which has no "outside" consequence compared to not using self at all, we see a steady memory consumption increase