Tasks should be allocated from a dedicated slab allocator cache

[Jakub Jermář]

The tasks management code could make use of the benefits offered by the slab allocator and allocate the task structures from a dedicated cache instead of malloc()'ing them. The most visible benefit would be the reuse of deallocated, but already initialized task structures trhough the use of constructor and destructor functions.

[Jakub Jermář]

Fixed in changeset:head,152.

As a further improvement in this area, an empty 'futexes' B+tree could be retained for constructed and task_destroy()'ed, but not yet destructed task_t instances.