Synchronize Measurements.getSummary() method to prevent ConcurrentModificationExceptions

[0xl3x1]

Previously the method Measurements.getSummary() wasn't set to synchronized, which was occasionally resulting in java.util.ConcurrentModificationException being raised when running YCSB with multiple threads. This commit fixes the issue by simply adding the synchronized keyword to Measurements.getSummary().

Let me know if you can think of any potential issues with making this change. I'm unsure if getSummary() was deliberately left unsynchronized for a reason?

[0xl3x1]

Similarly to #209, the build failure here is caused by #152, #181.

[busbey]

restarted CI build.

Won't this have a big perf impact?

[0xl3x1]

@busbey I haven't witnessed any major performance impact, but I could be missing something. As far as I can see, Measurements.getSummary() is only called here every 10 seconds.

[busbey]

that's a good point. There's also enough going wrong with synchronization across the rest of the class that this probably won't matter. hopefully all of this will get fixed by #214.

Doesn't exportMeasurements have the same problem?

[0xl3x1]

@busbey exportMeasurements may also have a synchronisation issue, but I haven't been using the exporter so am unsure and can't confirm the bug.

[busbey]

Filed #249 for the other method.

[busbey]

Thanks for the fix!