Blender Git Loki

Guarded allocator: Add explicit switch to the lockfree implementation

Previously the only way to use lockfree implementation was to start
executable and never switch to guarded allocator.

Surely, it is not possible to switch implementation once any allocation
did happen, but some tests are desired to test lock-free implementation
of the allocator. Those tests did not operate properly because the main
entry point of tests are forcing guarded allocator to help catching
bugs.

This change makes it possible for those tests to ensure they do operate
on lock-free implementation.

There is no functional changes here, preparing boilerplate for an
upcoming work on the allocator tests themselves.