Simplify the host's networking cleanup

[stevenengler]

This changes the host's net_ns: RefCell<Option<NetworkNamespace>> to net_ns: NetworkNamespace by adding a NetworkNamespace::cleanup function that has to be run while the worker has an active host. The network namespace objects have also been wrapped in a RefCell. The network interface objects don't have any &mut methods so this RefCell isn't currently needed, but we'll likely want to give them &mut methods in the future.

@robgjansen This can be merged either before or after your changes, whichever makes it easier for your PR.

[codecov]

Codecov Report

Base: 68.09% // Head: 67.46% // Decreases project coverage by -0.63% ⚠️

Coverage data is based on head (2348adc) compared to base (9021081).
Patch coverage: 82.05% of modified lines in pull request are covered.

❗ Current head 2348adc differs from pull request most recent head b280dba. Consider uploading reports for the commit b280dba to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2697      +/-   ##
==========================================
- Coverage   68.09%   67.46%   -0.63%     
==========================================
  Files         202      202              
  Lines       30388    30391       +3     
  Branches     5930     5933       +3     
==========================================
- Hits        20693    20504     -189     
- Misses       5001     5224     +223     
+ Partials     4694     4663      -31     

Flag	Coverage Δ
tests	67.46% <82.05%> (-0.63%)	⬇️

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

Impacted Files	Coverage Δ
src/main/host/descriptor/socket/inet/mod.rs	28.12% <0.00%> (ø)
src/main/network/net_namespace.rs	83.18% <76.00%> (-2.68%)	⬇️
src/main/host/host.rs	80.00% <100.00%> (-0.68%)	⬇️
src/main/host/network_interface.rs	76.92% <100.00%> (ø)
src/main/utility/mod.rs	54.34% <100.00%> (+1.01%)	⬆️
src/main/host/syscall/handler/eventfd.rs	0.00% <0.00%> (-68.43%)	⬇️
src/main/host/descriptor/eventfd.rs	0.00% <0.00%> (-65.66%)	⬇️
src/main/host/syscall/handler/fcntl.rs	47.29% <0.00%> (-17.57%)	⬇️
src/main/host/syscall/handler/ioctl.rs	40.00% <0.00%> (-17.15%)	⬇️
src/main/host/syscall/handler/unistd.rs	57.25% <0.00%> (-15.30%)	⬇️
... and 20 more

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