Is a manager of compiled query. It is responsible of CompiledQuery allocation
and could implement a caching policy. The identifier of a query is a
QueryDefinition.
author: S.Chassande-Barrioz
Retrieves a CompiledQuery for a given QueryDefinition. The compiled query
can be a new instance or not (caching).
Parameters: qd - defines the expected query. a CompiledQuery instance (never null).