Change the MemStore size accounting so we don't synchronize across three
volatiles applying deltas. Instead:
+ Make MemStoreSize, a datastructure of our memstore size longs, immutable.
+ Undo MemStoreSizing being an instance of MemStoreSize; instead it has-a.
+ Make two MemStoreSizing implementations; one thread-safe, the other not.
+ Let all memory sizing longs run independent, untied by
synchronize (Huaxiang and Anoop suggestion) using atomiclongs.
+ Review all use of MemStoreSizing. Many are single-threaded and do
not need to be synchronized; use the non-thread safe counter.
TODO: Use this technique accounting at the global level too.
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