Feature: Proxy mode: change proxy every N-th request

[exactstat]

If it's possible, please, add a possibility to change proxy every N-th request.
Add a variable (for setting N) and a new value for "Proxy mode" for this.

[Lusitaniae]

Some services like Luminati provide a single endpoint for all requests.

The right approach is to re establish the connection (after n connections, or in case of failure) to the same endpoint.

Currently, if the endpoint gets blocked, the crawl will be terminated because there's no more proxies available.

If I try to copy paste the proxy multiple times in the file, is still being aggregated in the code.