Improve content script performance by using browser cache properly

[ameshkov]

Issue Details

Currently, CoreLibs only partially uses cache as it ignores If-Modified-Since in the request headers.

Proposed solution

Here's what we should do:

1. Send Last Modified header in the response that is equal to the time when the filtering engine was built or changed last time.
2. Check If-Modified-Since in the requests and depending on its value return HTTP response with status 304 instead of trying to select cosmetic rules extra time.

Alternative solution

No response