`Seconds` type is too coarse

[endgame]

Some services (example: DynamoDB) generally respond really quickly (<10ms is normal, IME), and occasionally take much longer. A DynamoDB specialist advised us to give up on slow requests and retry them instead of waiting, but the Seconds type can only hold an integral number of seconds.

Replace it with either more granular integral type, or a type that can handle decimals?

[brendanhay]

Probably should switch to using DiffTime (or Pico directly.)

[endgame]

DiffTime sounds good, as it's a type about time, but will need to see what http-client is prepared to accept. But will think about that later.