Fix ConcurrentModificationException

[smoser]

14:04:18,026 INFO [ch.netzschmiede.flarmradar.srv.core.logbook.WaypointCache?] (EJB default - 7) Device=3C666E disappears from station=LSTB
Caused by: java.util.ConcurrentModificationException?

at java.util.HashMap?$HashIterator?.nextEntry(HashMap?.java:922) [rt.jar:1.7.0_75]
at java.util.HashMap?$ValueIterator?.next(HashMap?.java:950) [rt.jar:1.7.0_75]
at ch.netzschmiede.flarmradar.srv.core.logbook.WaypointCache?.analyzeCache(WaypointCache?.java:209)
at sun.reflect.GeneratedMethodAccessor43.invoke(Unknown Source) [:1.7.0_75]
at sun.reflect.DelegatingMethodAccessorImpl?.invoke(DelegatingMethodAccessorImpl?.java:43) [rt.jar:1.7.0_75]
at java.lang.reflect.Method.invoke(Method.java:606) [rt.jar:1.7.0_75]
at org.jboss.as.ee.component.ManagedReferenceMethodInterceptorFactory?$ManagedReferenceMethodInterceptor?.processInvocation(ManagedReferenceMethodInterceptorFactory?.java:72) [jboss-as-ee-7.1.1.Final.jar:7.1.1.Final]