Make use of QueryManager cache of compiled datastore queries

[andyjefferson]

The latest version of (org.datanucleus.store.query.)QueryManager allows storing and retrieval of compiled datastore queries. For example the new RDBMS query mechanism stores its compilation (the SQL, and mapping info for results and parameters) into this cache, and then retrieves it whenever a query needs compiling to check if we already have one compiled for that datastore. Would make sense for the LDAP plugin to make use of this.

What you store in the datastore-specific compilation object is up to you - e.g see the RDBMS case at
http://datanucleus.svn.sourceforge.net/viewvc/datanucleus/platform/store.rdbms/trunk/src/java/org/datanucleus/store/rdbms/query2/RDBMSQueryCompilation.java?revision=4998&view=markup
just cast the object returned by the QueryManager.getDatastoreQueryCompilation method.