Made a first use of btree_slice_t::init_superblock.
Removed some debugf lines.
Replaced another use of btree_slice_t::create with btree_slice_t::init_superblock.
Added last use of btree_slice_t::init_superblock, outside of unit tests.
Removed last use of btree_slice_t::create functions.
Removed superblock_id_ from btree_slice_t.
Removed the unused file containers/iterators.hpp.