server.py: get_latencies now reports percentiles _only_ if there are sufficient...

server.py:  get_latencies now reports percentiles _only_ if there are sufficient observations for the interpretation of the percentile to be unambiguous.
interfaces.py:  modified the return type of RIStatsProvider.get_stats to allow for None as a return value
NEWS.rst, stats.py: documentation of change to get_latencies
stats.rst: now documents percentile modification in get_latencies
test_storage.py:  test_latencies now expects None in output categories that contain too few samples for the associated percentile to be unambiguously reported.
fixes #1392