Add back a feature that was available in the old heap implementation
in release/v2.1 and earlier: keep track of which task allocates each
block from the heap. The task handle is conditionally added as
another word in the heap poisoning header under this configuration
option CONFIG_HEAP_TASK_TRACKING.
To allow custom monitoring and debugging code to be added, add helper
functions in multi_heap.c and multi_heap_poisoning.c to provide access
to information in the block headers.
