chore: Optimize garbage collection for Topology instance

[teogeb]

Extract crawl iteration run to a separate method. This way the local topology instance is cleaned up from memory immediately after the crawl.

Before this PR the topology instance was kept in memory until the start of the next iteration. This happens because the method is async, in a non-async method the topology would have been cleaned-up earlier (already when it execution exists the try-catch scope as it is a local variable there).

Other changes

Also simplified handling of subcriberGate. There is no need to pass it as an argument as it is a field of Crawler.