Fix concurrent usage of the login stack under heavy load

During performance test run with funkload, some errors occurs related to the usage of the LoginStack.
It seems concurrent threads are using the same LoginStack, which raises exception in the login module.

Example:

• At logout:

  ERROR [NuxeoAuthenticationFilter] Unable to logout java.lang.NullPointerException
  	at java.util.LinkedList.remove(LinkedList.java:791)
  	at java.util.LinkedList.removeLast(LinkedList.java:144)
  	at org.nuxeo.ecm.core.api.local.LoginStack.pop(LoginStack.java:46)
  	at org.nuxeo.ecm.core.api.local.ClientLoginModule.logout(ClientLoginModule.java:194)
  	at org.nuxeo.runtime.api.LoginModuleWrapper.logout(LoginModuleWrapper.java:92)
  

  or

• after login, to display a apge:

  Caused by: org.nuxeo.ecm.core.api.ClientException: Failed to load repository default
  at org.nuxeo.ecm.core.api.local.LocalSession.createSession(LocalSession.java:106)
  at org.nuxeo.ecm.core.api.local.LocalSession.getSession(LocalSession.java:166)
  at org.nuxeo.ecm.core.api.AbstractSession.connect(AbstractSession.java:213)
  at sun.reflect.GeneratedMethodAccessor87.invoke(Unknown Source)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  at java.lang.reflect.Method.invoke(Method.java:597)
  at org.nuxeo.ecm.core.api.TransactionalCoreSessionWrapper.invoke(TransactionalCoreSessionWrapper.java:94)
  at $Proxy21.connect(Unknown Source)
  at org.nuxeo.ecm.core.api.repository.Repository.open(Repository.java:155)
  at org.nuxeo.ecm.core.api.repository.Repository.open(Repository.java:113)
  at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.getTempCoreSession(DocumentModelImpl.java:432)
  at org.nuxeo.ecm.core.api.impl.DocumentModelImpl$RunWithCoreSession.execute(DocumentModelImpl.java:450)
  at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.loadDataModel(DocumentModelImpl.java:550)
  at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.getDataModel(DocumentModelImpl.java:564)
  at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.getProperty(DocumentModelImpl.java:717)
  at fr.dila.reponses.core.cases.ParapheurImpl.getQuestion(ParapheurImpl.java:32)
  ... 190 more
  Caused by: org.nuxeo.ecm.core.api.WrappedException: Exception: org.nuxeo.ecm.core.api.ClientException. message: Cannot create a core session outside a security context. You must login first.
  at org.nuxeo.ecm.core.api.local.LocalSession.createSession(LocalSession.java:94)
  ... 205 more