Investigate reducing the shared cache lock scope on commit

Per discussion thread on dev list [1] it should be possible to safely reduce the lock scope on the shared cache during commit to just the postprocessing phase following commit. The benefit is increased throughput. Theoretically this shouldn't cause any problems, but it requires some serious stress testing.

Another related idea is to do cache update postprocessing on both select and commit in a separate thread, again improving throughput. I am not assigning this to anyone. The task is up for grabs.

[1] http://objectstyle.org/cayenne/lists/cayenne-devel/2006/12/0026.html