Use old style string string formatting instead of string interpolation in performance critical places

[IgorFedchenko]

As per @Aaronontheweb :

One performance nitpick - better off using the old school string formatting syntax than string interpolation, because due to the way Akka.NET's logging is designed we can actually defer allocation of strings until we're ready to print the log statement out to console using the old technique. If the LogLevel is set at INFO, which is is by default, this saves us an unnecessary allocation each time. It can really add up in busy ActorSystems.