Memory allocations on enabling/disabling large filter lists

[ameshkov]

It seems that we're allocating too much memory when filter lists are enabled, disabled, or updated.

The problem is that golang does not "like" to free memory so we end up with high RSS. Ideally, we'd better avoid allocations whenever it's possible.

[szolin]

Maybe it's time for us to use Last-Modified HTTP header so we won't download any data unless the filter has been changed.