Implement emit after delay

[purplefox]

A push query can be suffixed with emit after delay , e.g. emit after delay 5s. In which case updates are not emitted every time the results change, but they are buffered and never emitted more than once in the specified time. This is useful, for example, if an aggregation (e.g. a sum of sales) is updated rapidly - once for each sale. But an end user only needs to know the results every few seconds.