ibuf_delete(): Note that the mtr must be committed before latching any
further pages.

ibuf_restore_pos(): New function, refactored from ibuf_delete_rec().

ibuf_merge_or_delete_for_page(): Commit the mini-transaction after
calling ibuf_delete().  If ibuf_restore_pos() fails, restart the loop.

rb://222 approved by Sunny Bains.  This should fix Issue #135.