webadmin: make pool queue data enum Java 8 compatible

The static method seems to cause problems for java 8 compilation.
Evidently there is something different about the availability of
of the enum instance reference inside the class definition.

But the better solution is just a shared instance method anyway.
Is there a need to change this in the backports? I imagine
systems won't be updating to java 8 unless installing 2.12 ...

Testing:

This compilation error goes away with the patch:

[ERROR] /Developer/Work/fnal/dCache/dcache/modules/dcache-webadmin/src/main/java/org/dcache/webadmin/model/businessobjects/rrd4j/PoolQueuePlotData.java:[90,24] no suitable method found for getSourceName(<anonymous org.dcache.webadmin.model.businessobjects.rrd4j.PoolQueuePlotData.PoolQueueHistogram>)
method org.dcache.webadmin.model.businessobjects.rrd4j.RoundRobinDatabaseDatasource.getSourceName() is not applicable
(actual and formal argument lists differ in length)
method <anonymous org.dcache.webadmin.model.businessobjects.rrd4j.PoolQueuePlotData$PoolQueueHistogram$1>.getSourceName() is not applicable
(actual and formal argument lists differ in length)

Target: master
Patch: https://rb.dcache.org/r/7370
Acked-by: Gerd
Require-book: no
Require-notes: no

Note that the pull requests for rb 7346 have been updated to take care of this.