WL#854: Mapped HEAP tables.

In some tasks we would like to have data to be accessed very fast, with guaranty
no Disk IO will be needed, While would not like data to be lost on MySQLD restart.

This can be archived by mapping HEAP table to some of persistent tables, so what
all updates to this HEAP table will update persitent able as well. Also we need
this heap table to be loaded from persistant table on the first open.

The genetic syntax is:

create table t1 (...) type=HEAP MAPPED_TO=persistent_table.

Alternatively can just add it as an option for existent tables:

create table t1 (...) HEAP_MAPPED=1;

Which will result in shadow table created for such table.