change overlap, stride in adapter config from samples to offsets

AdapterConfig previously accepted overlap and stride in terms of samples, now it is changed to offsets, so that we can support different sample rates on different pads. One example is the FrameWriter.

One can convert samples or duration to offsets simply by using the tools in the Offset class.

I chose offsets instead of duration because the Correlate element requires padding of filter_length - 1 samples, and sometimes that doesn't map to integer nanoseconds.

One option we can choose is to allow one AdapterConfig per pad, but I think that will give users the impression that the streaming behavior can be different pad by pad, which defeats the purpose of using the _TSTransSink base class, whose job is to synchronize pads.

Another option is we can let the user either input offsets or duration, but that also might lead to some confusion(?)