A fix and a test case for Bug#1051006

A fix and a test case for Bug#1051006
"Tree iterators return garbage if an index is modified between calls"

Mark in a deleted node in sptree.h that it's been  put into the
garbage heap. When iterting over a garbage collected node, skip it,
and go up the stack until we find the first valid node.

This breaks the "sorted" quality of tree iterators in case there
are modifications between invocations of an iterator:
it is possible that a node is deleted and recycled, and we don't see
it in the iterator. When we go up the stack, we can jump to a different
part of the range than the one the recycled node belongs to.
. With this fix, it is also possible, that the iteration goes more
than once over entire tree range. But it's a good enough quick fix for a
crashing expire loop, which uses the tree iterator over the primary key to
scan the entire range and deletes expired keys on the go (additionally,
deletions may occur between invocations of the expire loop).