Need suggestions for time scheduling async tasks

[arnetheduck]

The classic approach to rate limiting is via the use of a [token 
bucket](https://en.wikipedia.org/wiki/Token_bucket) \- here's an implementation 
for `chronos` (asyncdispatch alternative): 
<https://github.com/status-im/nim-chronos/blob/master/chronos/ratelimit.nim#L126>