Prepared for upcoming change to cdb_next_item() API.

Prepared for upcoming change to cdb_next_item() API.

cdb_next_item() is always preceded by a call to cdb_rewind() before the
iteration loop.  We're going to remove the cursor from thread-specific
data, and replace it with a void pointer that the caller is responsible
for passing back.  This will make the backend drivers easier to build.