Fix ConcurrentModificationException for files actions

[v1v]

Synchronised the files list to prevent the build serialisation job to access it and throw ConcurrentModificationException .

What did I try?

• newConcurrentHashSet

I used newConcurrentHashSet rather than the newHashSet method from the google-http-client dependency since it does not support concurrentHash set as far as I see

but it failed when deploying it in one of my production jenkins instance

java.lang.NoSuchMethodError: com.google.common.collect.Sets.newConcurrentHashSet()Ljava/util/Set;

Closes #61

[v1v]

This particular methods doesn't seem to work in our existing instance

java.lang.NoSuchMethodError: com.google.common.collect.Sets.newConcurrentHashSet()Ljava/util/Set;
19:56:27  	at com.google.jenkins.plugins.storage.reports.BuildGcsUploadReport.<init>(BuildGcsUploadReport.java:38)
19:56:27  	at com.google.jenkins.plugins.storage.reports.BuildGcsUploadReport.of(BuildGcsUploadReport.java:74)
19:56:27  	at com.google.jenkins.plugins.storage.AbstractUpload.perform(AbstractUpload.java:173)
19:56:27  	at com.google.jenkins.plugins.storage.ClassicUploadStep.perform(ClassicUploadStep.java:183)
19:56:27  	at org.jenkinsci.plugins.workflow.steps.CoreStep$Execution.run(CoreStep.java:80)
19:56:27  	at org.jenkinsci.plugins.workflow.steps.CoreStep$Execution.run(CoreStep.java:67)
19:56:27  	at org.jenkinsci.plugins.workflow.steps.SynchronousNonBlockingStepExecution.lambda$start$0(SynchronousNonBlockingStepExecution.java:47)
19:56:27  	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
19:56:27  	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
19:56:27  	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
19:56:27  	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
19:56:27  	at java.lang.Thread.run(Thread.java:748)
19:56:27  Finished: FAILURE