(r2962, r2865, r2864) and do not refuse to buffer deletes.  The
problem with refusing to buffer deletes was that no deletes were
buffered.

Instead, when merging delete requests, disregard a delete request
if it would make the page empty.  InnoDB tolerates redundant secondary
index records, but not empty pages.

ibuf_delete(): When merging a delete request, print and ignore the record
if the page would become empty.