Rewrite MPFuture to use native SharedMemory

[mryab]

This PR rewrites hivemind.utils.MPFuture to use native SharedMemory from Python standard library, available since Python 3.8. As a result, we can reduce the codebase's reliance on PyTorch and make MPFuture easier to maintain

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 21 lines in your changes missing coverage. Please review.

Project coverage is 85.19%. Comparing base (d20e810) to head (4bb054f).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
hivemind/utils/mpfuture.py	74.64%	18 Missing ⚠️
hivemind/p2p/p2p_daemon.py	87.50%	2 Missing ⚠️
hivemind/p2p/p2p_daemon_bindings/control.py	97.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   85.39%   85.19%   -0.21%     
==========================================
  Files          81       96      +15     
  Lines        8006     8619     +613     
==========================================
+ Hits         6837     7343     +506     
- Misses       1169     1276     +107     

Files with missing lines	Coverage Δ
hivemind/utils/asyncio.py	99.27% <100.00%> (-0.73%)	⬇️
hivemind/p2p/p2p_daemon_bindings/control.py	92.16% <97.36%> (+0.56%)	⬆️
hivemind/p2p/p2p_daemon.py	95.96% <87.50%> (+0.38%)	⬆️
hivemind/utils/mpfuture.py	88.84% <74.64%> (-4.50%)	⬇️

... and 30 files with indirect coverage changes

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