Avoid synchronization on GetQueues

[klockla]

The purpose of this PR is to avoid the synchronization on the internal queue map.

The synchronized(getQueues()) call causes contention during long queue traversals which are very likely to occur during calls to countURLs & ListURLs.

This change is based on the ConcurrentOrdereredMap class which uses StampLock to provide fine
fine-grained locking over the concurrent map.

(alternative in ConcurrentLinkedHashMap based on ReentrantLock, need to compare under stress tests)

Signed-off-by: Laurent Klock [email protected]

[jnioche]

Thanks @klockla, sounds good
Would be great to have a test or benchmark to compare the existing approach vs ConcurrentOrdereredMap vs ConcurrentLinkedHashMap indeed.
Given that the same approach is used in key parts of StormCrawler it would be great to have a better approach.
Is the code for ConcurrentOrdereredMap and ConcurrentLinkedHashMap from another project or did you write that? We would need a strong set of unit tests to make sure they are bullet proof