qa: make closure of NEOCluster more reliable in treaded tests
Instances of NEOCluster were not deleted as soon as the only referrers were weak proxies (at least that's what a quick check with the 'gc' module showed at the beginning of tearDown). In some cases, __del__ was called while the next test was logging a message, which led to deadlocks. Without those proxies, it may be reliable, but only on CPython. See http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies Relying on __del__ to close a cluster was wrong. NEOCluster is now a context manager that does it explicitly at exit, in addition to automatically stop it. The NEOCluster.stop method combines the previous stop/__del__/reset methods. A new 'with_cluster' decorator is also added to avoid excessive indentation in tests. Unindentation of existing tests will be done later.
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