WL#5499: MEMORY storage engine: optimize DELETE queries

There are reports that DELETE from MEMORY tables with indexes are inexcusable slow.

One of the reports is BUG#30090 - DELETE from MEMORY table with BTREE primary
key extremely slow. Closed as duplicate of BUG#30590.

There is another report by Mat Keep and Mikael Ronstrom based on sysbench and
oprofile.

Since information is rather contradictory and incomplete, it is not yet clean
what exactly has to be improved. But it is definitely worth to be analysed.