Use pointer-based thread identifiers instead of number-based ones. This
should resolve compilation warnings on BSD systems.

Linux uses integers as thread identifiers, BSD based systems use pointers.

ref #157

ref #157

ref #157

Refs #155

 - Applicable when using very low signature lifetimes

Refs #155, but does not resolve it

This is only temporary solution until the proper rewrite.

refs #154

There is a merge request waiting for this.

This reverts commit 25e054c4.