Batching accumulate to avoid wasteful re-propagations

From irc:

<conan> the accumualte needs to be batched
<conan> imagine you have an accumulate follows by another node
<conan> if you do 10 assertions
<conan> what you want to do is aggregate over the 10 assertions
<conan> and then propagate it to the next node
<conan> what actually happens is each time the accumulate is true, for each assertion
<conan> it retracts all hild propagations
<conan> re-calculates the aggregate
<conan> and re-propagates
<conan> which is terribly wasteful