threshold for dummy event sending should be configurable

we're spending quite a lot of effort doing state res when we get typing notifications in large rooms with 10 extremities. hacking the code (https://github.com/matrix-org/synapse/compare/14bb4cff99ddd8b5eb1816c5405d52a8f13e29d6...1d41815b0980a6c9d6a581e78d2606a41d1d4485) to lower the threshold seemed to improve things. Please can it be configurable?