avoid forcibly interrupting JE thread (#157)

When LDIF is invalid, for example:

```
version: 1

dn: dc=test2,dc=cn
objectclass: domain
objectclass: top
dc: test                <--- wrong, should be "test2"

dn: ou=groups,dc=test2,dc=cn
objectClass: organizationalUnit
objectClass: top
ou: groups

```

This will trigger `DnValidationCursorDecorator.throwIfOrphan(dn)`,
make `OnDiskMergeImporter.waitTaksTermination()` exit early and
`OnDiskMergeImporter.invokeParallel()` immediately call `executor.shutdownNow()`,
this leads to this exception and disabled JE backend:

```
  category=TOOLS severity=ERROR msgID=96 msg=An error occurred while attempting to process the LDIF import:  ThreadInterruptedException((JE 18.3.12) Environment must be closed, caused by: com.sleepycat.je.ThreadInterruptedException: Environment invalid because of previous exception: (JE 18.3.12) /home/opendj/db/userRoot java.lang.InterruptedException THREAD_INTERRUPTED: InterruptedException may cause incorrect internal state, unable to continue. Environment is invalid and must be closed.)
  category=CONFIG severity=ERROR msgID=646 msg=org.forgerock.opendj.config.server.ConfigChangeListenerAdaptor.applyConfigurationChange failed for entry ds-cfg-backend-id=userRoot,cn=Backends,cn=config: result code=Other, admin action required=false, messages="An error occurred while trying to initialize a backend loaded from class org.opends.server.backends.jeb.JEBackend with the information in configuration entry ds-cfg-backend-id=userRoot,cn=Backends,cn=config: InitializationException: The database environment could not be opened: com.sleepycat.je.ThreadInterruptedException: (JE 18.3.12) Environment must be closed, caused by: com.sleepycat.je.ThreadInterruptedException: Environment invalid because of previous exception: (JE 18.3.12) /home/opendj/db/userRoot java.lang.InterruptedException THREAD_INTERRUPTED: InterruptedException may cause incorrect internal state, unable to continue. Environment is invalid and must be closed. (BackendImpl.java:972 BackendImpl.java:179 BackendConfigManager.java:1063 BackendConfigManager.java:1045 BackendConfigManager.java:275 BackendConfigManager.java:887 BackendConfigManager.java:77 ServerManagedObjectChangeListenerAdaptor.java:50 ConfigChangeListenerAdaptor.java:281 ConfigurationHandler.java:648 ConfigurationBackend.java:497 LocalBackendModifyOperation.java:493 LocalBackendModifyOperation.java:280 LocalBackendWorkflowElement.java:620 LocalBackendWorkflowElement.java:743 ModifyOperationBasis.java:337 InternalClientConnection.java:1322 InternalClientConnection.java:1404 TaskUtils.java:205 TaskUtils.java:173 ImportTask.java:700 ...). This backend will be disabled"
 ```