Use std::chrono in polling calculations

We have five different types to represent time in Tango (time_t, timeval, long (ms), double (s), Tango::TimeVal) and we convert between them a lot in the whole codebase. Sometimes we also need to shift timestamps by DELTA_T to avoid overflows. All this is error-prone and requires a lot of duplicated code.

This changes polling thread and related classes to operate on std::chrono durations and time_points .

Follow-up of #704. Fixes #675 (closed).