Dropped usage of fiber memory pools in tests
Marked fiber memory pool APIs as deprecated.

It turns out that client application is likely to have it's own
memory pools which suit it's needs. Providing a default one, based on
linked list and malloc/free, seems out of scope of this library.

Newer destructors API can cover all required use cases when something
needs to be done when fiber is removed (e.g. clear some client provided
memory pool, or free all the required memory manually).

These APIs will be removed soon.