Timeline query doesn't work across datasets

As posted by @jacob.golomb on https://github.com/gwpy/gwpy/issues/1527:

There seems to be a bug when grabbing a list of segments from O3 that includes O3a and O3b. Finding the active segments from a time period that includes all of O3 gives a segment list that ends at the end of O3a:

>>> from gwpy.segments import DataQualityFlag
>>> DataQualityFlag.fetch_open_data('V1_DATA', 'Apr 1 2019', 'Mar 27 2020').active.coalesce()[-1]
Segment(1253944467, 1253977218)
>>> gwpy.time.from_gps(1253977218)
datetime.datetime(2019, 10, 1, 15, 0)

This indicates that the final segment ends in October 2019, whereas I would expect the final segment to end in March 2020.

The fundamental issue is that the gwosc.timeline.get_segments function only returns data from one dataset at a time. I'm not sure the best way to attempt to aggregate data from multiple datasets, which may be related (e.g. O3a_4KHZ_R1 vs O3b_4KHZ_R1) but could be completely independent (O3a_4KHZ_R1 vs O3a_16KHZ_R1).

@martin.beroiz, any ideas?