Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/api/DataEngineContext.java
{
	
	public final static int MODE_GENERATION = 1;
	public final static int MODE_PRESENTATION = 2;
	public final static int DIRECT_PRESENTATION  = 3;
	
	/**
	 * this is a special mode, that a query which is running on a report
	 * document also needs to be stored into the same report document. This is a
	 * update operation, and it is a combination mode of presentation and
	 * generation. One note is in running in this mode, the report document must
	 * be same for reading and writing.