Rejigger the the cell backup so it blocks the ioloop less

[jnwatson]

No description provided.

[vEpiphyte]

Tests broken due to jupyter/jupyter_client#637

[codecov]

Codecov Report

Merging #2145 (b2855c0) into master (96521d9) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2145      +/-   ##
==========================================
- Coverage   97.11%   96.97%   -0.14%     
==========================================
  Files         154      154              
  Lines       31207    31203       -4     
==========================================
- Hits        30306    30260      -46     
- Misses        901      943      +42     

Flag	Coverage Δ
linux	96.97% <100.00%> (-0.14%)	⬇️
linux_replay	?

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

Impacted Files	Coverage Δ
synapse/lib/cell.py	97.88% <100.00%> (-0.02%)	⬇️
synapse/lib/nexus.py	96.99% <100.00%> (+0.02%)	⬆️
synapse/tests/utils.py	93.77% <0.00%> (-2.12%)	⬇️
synapse/lib/hiveauth.py	96.18% <0.00%> (-0.73%)	⬇️
synapse/cortex.py	96.20% <0.00%> (-0.71%)	⬇️
synapse/lib/view.py	94.18% <0.00%> (-0.70%)	⬇️
synapse/lib/trigger.py	94.11% <0.00%> (-0.43%)	⬇️
synapse/lib/layer.py	98.11% <0.00%> (-0.06%)	⬇️
synapse/lib/hive.py	98.31% <0.00%> (+0.24%)	⬆️

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