Improve resource management for network sessions

[samschott]

This PR introduces manual scoping of network sessions.

This is used to create a new network session and clean up its resources for:

1. Individual upload or download threads from our thread pool.
2. Entire upload or download sync cycles.
3. Main API calls that may use significant network resources.

This improves memory usage for long running daemons by more frequently and immediately cleaning up network resources. It may also work around a memory issue with requests when used in threads, see psf/requests#4601 (comment).

[codecov]

Codecov Report

Merging #340 (e395205) into master (acf6d4f) will increase coverage by 0.12%.
The diff coverage is 88.72%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
+ Coverage   72.07%   72.19%   +0.12%     
==========================================
  Files          30       30              
  Lines        6324     6363      +39     
==========================================
+ Hits         4558     4594      +36     
- Misses       1766     1769       +3     

Flag	Coverage Δ
pytest	72.19% <88.72%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
src/maestral/main.py	64.10% <28.57%> (-0.30%)	⬇️
src/maestral/sync.py	84.57% <95.09%> (+0.21%)	⬆️
src/maestral/client.py	56.74% <100.00%> (+0.87%)	⬆️

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[samschott]

See #248.