FT-300 Add 'heat' to the block allocator API, which is a hint for how
likely the allocation will need to move again at the next checkpoint (we
pass the node height for this value). The new heat zone allocation
strategy uses the heat value to put nonleaf nodes towards the end of the
file and leaf nodes towards the beginning.
FT-279 Clean up ftnode_fetch_extra struct and, most importantly, its
initialization code