Fix the 'CompassBackend session is null' error in asynchronous indexing

When the IndexingThreadImpl is recycled, the Compass session is not reinitialized. This leads to the following error when trying to index a document :

org.nuxeo.ecm.core.search.api.client.IndexingException: CompassBackend session is null for thread Thread[Thread-31,5,JBoss Pooled Threads]
at org.nuxeo.ecm.core.search.backend.compass.CompassBackend.index(CompassBackend.java:369)
at org.nuxeo.ecm.core.search.service.SearchServiceImpl.index(SearchServiceImpl.java:285)
at org.nuxeo.ecm.core.search.service.SearchServiceImpl.index(SearchServiceImpl.java:296)
at org.nuxeo.ecm.core.search.threading.task.IndexingSingleDocumentTask.run(IndexingSingleDocumentTask.java:87)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
at java.lang.Thread.run(Thread.java:595)