beam eats cpu on low mutation rate (was: couchdb separation)

While doing some testing in Amazon, I noticed beam.smp reporting usage of nearly 50% of CPU with no views, no xdcr, and no compaction running.

The load was about 25k gets and 25k sets per second, writes were all updates to the same ~17k items.

After some investigation by Alk, it was determined that this is likely due to the load being sufficiently low to persist one or two docs per vbucket per flushing round. And every vbucket commit has to synchronously notify couchdb about updated header position.

The CPU usage may be "harmless", but it will appear to users to be quite high, and at some point, impact the performance of the system as a whole.

This bug is to raise awareness and track any improvements we can make.