PIPE-3073: parallelize importdata QA flagdata calls; fix xvfb DISPLAY propagation
- daskhelpers: add parallel_map() utility that runs fn(item) for each
item via dask.delayed/dask.compute when is_daskclient_allowed(), else
falls back to sequential map()
- importdata/qa: use parallel_map() to run flagdata(mode='summary')
across all EBs concurrently; replace serial for-loop with zip() over
pre-fetched results; no change to downstream logic
- __init__.start_xvfb(): switch Display(visible=0) to
Display(backend='xvfb', manage_global_env=True); explicitly set
os.environ['DISPLAY'] from new_display_var after .start() to
guarantee child processes (e.g. casaplotserver/Qt) inherit the
correct virtual display rather than the SSH-forwarded :0; include
effective DISPLAY value in debug log
