Add unit tests.

Little refactoring +xlog, +snap, snapshot->snap

Be more verbose when exiting the spawner.

Implementation of TreeIndex::reserve() had a bug when
tree.members weren't allocated while buildNext was already
using them.

Remove sptree_index_node magic and simplify sptree iterators.

                            </kostja>

Conflicts:
	src/box/tree_index.cc

When tree.max_size is 0, tree.max_size * 2 is also 0.