The blocking executor has been deprecated in Pike and marked for removal
in Rocky, but some user like Mistral asked us to wait before. We decided
to remove this executor for Train or next cycle, now we are in the
Ussuri and after some researchs on usage I think we can go ahead.

This patch drop the deprecation warnings, related unit tests and
set the server with the threading executor is the default executor.

Change-Id: If07bab61ee2b148658b88be98b12f8539f274efe
Closes-Bug: #1715141