Fix NPE in httpd service

The patch fixes the following NPE:

17 nov. 2014 11:52:48 (poolCollector) [] RuntimeException while executing runnable com.google.common.util.concurrent.Futures$5@2437e8e6 with executor com.google.common.util.concurrent.MoreExecuto
rs$SameThreadExecutorService@3f726b8a
java.lang.NullPointerException: null
        at java.util.TreeMap.rotateLeft(TreeMap.java:2220) ~[na:1.8.0_25]
        at java.util.TreeMap.fixAfterInsertion(TreeMap.java:2287) ~[na:1.8.0_25]
        at java.util.TreeMap.put(TreeMap.java:582) ~[na:1.8.0_25]
        at diskCacheV111.services.web.PoolCellQueryContainer.put(PoolCellQueryContainer.java:57) ~[dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at diskCacheV111.services.web.PoolInfoObserverV3$2.onSuccess(PoolInfoObserverV3.java:140) ~[dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at diskCacheV111.services.web.PoolInfoObserverV3$2.onSuccess(PoolInfoObserverV3.java:134) ~[dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at com.google.common.util.concurrent.Futures$5.run(Futures.java:1231) ~[guava-17.0.jar:na]
        at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:297) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.ExecutionList.executeListener(ExecutionList.java:156) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:145) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.AbstractFuture.set(AbstractFuture.java:185) [guava-17.0.jar:na]
        at org.dcache.cells.CellStub$CallbackFuture.set(CellStub.java:594) [dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at org.dcache.cells.CellStub$CallbackFuture.answerArrived(CellStub.java:616) [dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$CallbackTask.innerRun(CellNucleus.java:1050) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:1005) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:297) [guava-17.0.jar:na]
        at dmg.cells.nucleus.CellNucleus.addToEventQueue(CellNucleus.java:813) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellGlue.sendMessage(CellGlue.java:557) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus.sendMessage(CellNucleus.java:371) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:831) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$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 java.lang.Thread.run(Thread.java:745) [na:1.8.0_25]

The patch also changes a few return types from Set to List, as the code
is more concerned about the ordering that uniqueness (and the items in
the list are still unique). Changed a TreeMap to a more efficient
HashMap.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
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/7510/
(cherry picked from commit 5ee36f25234a07747e0d1b90de72141f7abdbbd8)
Merge pull request #1095 from gbehrmann/fix/2.11/rb7510

Fix NPE in httpd service