With innodb_change_buffering enabled, Innodb buffers
all modifications to secondary index leaf pages when
the leaf pages are not in buffer pool.

Crash InnoDB while an IBUF_OP_DELETE is being applied.
Restart and note that the same record can be applied
again which may lead to crash.

Mark the change buffer record processed, so that it will
not be merged again in case the server crashes between
the following mtr_commit() and the subsequent mtr_commit()
of deleting the change buffer record.

Testcase: No testcase because it is difficult to get the
timing right with the two asyncronous task purge and change
buffering

Approved by Marko. rb#1893