Use Dynamic Row Format for implicit temporary tables

Percona Server has implemented dynamic row format for MEMORY/HEAP engine, but it's not used for implicit temporary tables and thus queries reading TEXT/BLOCK columns will still end up going to disk. Vadim mentioned that this had been considered before but was finally never looked into (and Laurynas said "One quite important feature is missing from this release however: MyISAM tables are still used for temporary tables where the MEMORY tables with dynamic row format could be used instead: for schemas containing large VARCHARs, BLOB, TEXT. Unfortunately, enabling this feature is not trivial, but we are working on this and expect to deliver it in a future release.")

The requested feature is precisely that: when a query or admin command (SHOW COLUMNS for example) will create an on-disk temporary table, conditions will be evaluated according to restrictions described here https://www.percona.com/doc/percona-server/5.5/flexibility/improved_memory_engine.html?id=percona-server:features:improved_memory_engine#restrictions and if the implicit temporary table complies to requirements, then it will use HEAP engine.