F# code cleanup

* replace unused self variables to `_`
* `printf`-powered `Logging` functions
* remove unnecessary `ActorPath` type extensions
* replace `xxx :> SomeType` with `upcast xxx`
* remove unnecessary `new`
* remove unnecessary `'` variable suffix 
* replace `match o with :? 'T -> f (o :?> 'T)` to `match o with :? 'T as t -> f t` and the like