Issue 110 : bulkOperations integration tests

Several things to review here :
* the integration tests of course
* some rewrites of the putAll and removeAll methods (with some refactorings too)
Thanks for your input in advance
You can either remove this test because it relied on the previous implementation of `bulkComputeIfAbsent()` seeing a mutation to the same key that took place after the compute was finished, or revert the implementation to the previous one. Despite what you guys might have thought, it wasn't actually broken because of the final `putIfAbsent` call on the store, that would not install computed value unless it was still absent (i.e. no racing mutation occurred in the meantime).