Follow-up from "Search disk files multithreaded"

The following discussions from !208 (merged) should be addressed:

• @cullmann started a discussion:

  Aren't these initializer duplicates of the ones above or the C++11 initializers in the header?

• @cullmann started a discussion:

  If the destructor is just the default one, I would remove the implementation.

• @cullmann started a discussion:

  As said in the general comment: I would tend to still either just go for ideal thread count or if that is measurable worse on your machine default to ideal thread count but provide some setting for this.