This is needed to avoid arbitrarily large blowup
for many-element windows.  Now, with a fixed
probability (currently 0.2%) when writing the state,
instead of doing a blind write we read in all
accumulated values, combine, and replace everything
with the final combined value.

----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=109239045