Fix NPE in cell initialization

Cells block delivery of messages until they have been started. Prior to being
started the message is returned to the sender. The code however suffers from a
race resulting in the following exception:

17 nov. 2014 15:01:17 (SrmSpaceManager) [] Uncaught exception in thread SrmSpaceManager-0
java.lang.NullPointerException: null
        at dmg.cells.nucleus.CellAdapter.getCellName(CellAdapter.java:325) ~[cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:784) ~[cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$DeliverMessageTask.innerRun(CellNucleus.java:1095) ~[cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:1005) ~[cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_25]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_25]
        at dmg.cells.nucleus.CellNucleus$3.run(CellNucleus.java:696) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at java.lang.Thread.run(Thread.java:745) [na:1.8.0_25]

The cause is the classic problem of leaking a reference to an object in its
constructor. In this case CellNucleus registers itself with CellGlue inside
its constructor, meaning that others may deliver messages to it even before
the final nucleus member in CellAdapter has been initialized.

This minimal patch moves the CellGlue registration into a new CellNucleus#start
method. The method is called right after the constructor returns. The patch
is kept small to allow it to be backported, however a subsequent patch for trunk
will change the code further to not start the CellNucleus until the cell has
been started, thus solving the problem of early message delivery.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7511/
(cherry picked from commit 019e10e1eef0469e447e4a80bde1a8e708ae2de3)