Bug background capture

[vEpiphyte]

• Background command syn:task captured the AST and not the original query.
• Background command variable capture could capture heavy objects, breaking ps commands.

For background command from #1931

[codecov]

Codecov Report

Merging #1957 (f779a75) into master (637c061) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1957      +/-   ##
==========================================
- Coverage   96.73%   96.49%   -0.24%     
==========================================
  Files         142      142              
  Lines       26956    26959       +3     
==========================================
- Hits        26075    26015      -60     
- Misses        881      944      +63     

Flag	Coverage Δ
linux	96.49% <100.00%> (-0.24%)	⬇️
linux_replay	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
synapse/lib/storm.py	98.12% <100.00%> (+<0.01%)	⬆️
synapse/lib/spawn.py	89.74% <0.00%> (-6.74%)	⬇️
synapse/tests/utils.py	93.56% <0.00%> (-2.20%)	⬇️
synapse/cortex.py	95.52% <0.00%> (-0.79%)	⬇️
synapse/lib/hiveauth.py	95.96% <0.00%> (-0.77%)	⬇️
synapse/lib/view.py	95.37% <0.00%> (-0.73%)	⬇️
synapse/lib/trigger.py	93.91% <0.00%> (-0.44%)	⬇️
synapse/lib/lmdbslab.py	99.39% <0.00%> (-0.21%)	⬇️
synapse/lib/layer.py	97.80% <0.00%> (-0.07%)	⬇️

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