#1959

Refetch MemoryUsage each time we use it.

Note:
```ant clean test``` pass
```mvn clean install``` fails with AND without this patch but I don't understand why :

```
Creating C:\Users\sys\git\orientdb\graphdb\target\surefire-reports\GraphDB test
suite\Default.xml
FAILED: testImport
java.lang.NoSuchMethodError: com.orientechnologies.orient.core.type.tree.OMVRBTr
eeRID.getLock()Lcom/orientechnologies/common/concur/resource/OSharedResourceAdap
tiveExternal;
        at com.tinkerpop.blueprints.impls.orient.OrientVertex.addEdge(OrientVert
ex.java:226)
....
```