Make it possible to specify interval as float which allows to use also fractions of a second.

It is usable for activity nodes where we need activity processing (i.e. tic) to happen much faster that normally (default is 5 seconds).

It has been successfully used in a production system for months (and big dev instance) despite not done in generic line now proposed.

@nexedi , any objections for not having this in a generic ERP5's timerservice?