rdtech-um: concentrate poll period handling in one spot

rdtech-um: concentrate poll period handling in one spot

The handling of "command sent at ..." and the emission of periodic poll
requests is non-trivial, and was scattered around different locations.
Concentrate all of the time scaling and delta calculation and updating
internal state into a single spot, and allow callers to force the first
emission when desired (acquisition start needs this). Which eliminates
repeated calls into the microseconds getter.

Not advancing the last checkpoint, and keep sending requests until
successful transmission, remains identical to the previous version of
the implementation. In practise transmit errors are rare.