cells: Fix race conditions in cell lookup

When cells are killed, they are moved from one map to another map. This operation
is not atomic and methods that look in both places need to be synchronized.

This addresses a null pointer exception experienced in the DCAP door:

java.lang.NullPointerException: null
        at diskCacheV111.doors.DCapDoorInterpreterV3.ac_get_door_info(DCapDoorInterpreterV3.java:733) ~[dcache-dcap-2.2.13.jar:2.2.13]
        at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source) ~[na:na]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_19]
        at java.lang.reflect.Method.invoke(Method.java:601) ~[na:1.7.0_19]
        at dmg.util.CommandInterpreter.execute(CommandInterpreter.java:609) ~[cells-2.2.13.jar:2.2.13]
        at dmg.util.CommandInterpreter.command(CommandInterpreter.java:486) ~[cells-2.2.13.jar:2.2.13]
        at dmg.cells.nucleus.CellAdapter.executeLocalCommand(CellAdapter.java:1020) ~[cells-2.2.13.jar:2.2.13]
        at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:940) ~[cells-2.2.13.jar:2.2.13]
        at dmg.cells.nucleus.CellNucleus$DeliverMessageTask.innerRun(CellNucleus.java:1051) ~[cells-2.2.13.jar:2.2.13]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:949) ~[cells-2.2.13.jar:2.2.13]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_19]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_19]
        at dmg.cells.nucleus.CellNucleus$1.run(CellNucleus.java:654) [cells-2.2.13.jar:2.2.13]
        at java.lang.Thread.run(Thread.java:722) [na:1.7.0_19]

Target: trunk
Request: 2.6
Request: 2.2
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7896
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5700/
(cherry picked from commit 45850f5d50d867772d91b8526d9b7210ae9aaa55)