Consider lifting Send+Sync restrictions and migrating to a single thread tokio runtime

[let4be]

We could simplify && speed things up
=> spawn several Crawlers in their own threads, we already handle job delegation via channels
this way we have less internal/external Send/Sync restrictions and we do not bother tokio with work stealing schenanigans which most likely hurt more than help in our use case

after this is implemented, can review let4be/crusty#35
probably could use Actix here and there