Clustering performance regression in scenarios with ASYNC cache used

Our clustering write-heavy stress tests show a performance drop of 7.0.0.ER2 bits compared to 6.4.0 bits, when using ASYNC cache.

Compare these results:

stress-ejbremote-dist-async
7.0.0.ER2: throughput
6.4.0.GA: throughput

stress-ejbremote-repl-async
7.0.0.ER2: throughput
6.4.0.GA: throughput

stress-ejbservlet-repl-async
7.0.0.ER2: throughput
6.4.0.GA: throughput

stress-session-repl-async
7.0.0.ER2: throughput
6.4.0.GA: throughput

stress-session-dist-async
7.0.0.ER2: throughput
6.4.0.GA: throughput

Also, load scenarios, which use a stable load of 4000 sessions are affected.