Add method to BasicCrawler to handle just one request.

Add statistics

TODO:
Make mypy happy about statistics.
Wrap existing statistics from init in adaptive statistics.
Silent subcrawler statistics and loggers in general. (Set level to error?)

Ignore and create TODO follow up issue for refactoring Statistics class after technical discussion.

Pre-navigation hooks delegation to sub crawler hooks.

Statistics were marked as generics, but in reality were not.
Hardcoding state_model to make it explicit and clear.

WIP KVS handling. Currently it does not go through Result handler.