Rethink memtable size-tracking strategy

[Terkwood]

Calling Memtable.Sizer.resize(key, value) in the update method of the web controller takes some time. It takes even longer when the system is under heavy load. Under 500 user load, the Resize call (simply a cast to a genserver!) can take half the time of the update itself.

We may be able to make the PUT route faster by recalculating the size of the memtable once every few seconds.