INSERT BUFFER MERGE

Problem:

When the record is merged from the change buffer to the actual page,
in a particular condition, it is assumed that the deleted rec will
be re-used by the inserted rec.  With this assumption the lock is
restored on the pointer to the deleted rec itself, thinking that
it is pointing to the newly inserted rec.

Solution:

Just before restoring the lock, update the rec pointer to point
to the newly inserted record.  An assert has been added to verify
this.  This assert will fail without the fix and will pass with
the fix.

rb#2449 in review by Marko and Jimmy