SYN-5967: Convert yaml.safe_load to yaml.load(..., yaml.CSafeLoader)

[MichaelSquires]

• Switch to opportunistically using yaml CSafeLoader instead of the pure-python SafeLoader (via safe_load)
• Add synapse.common.yamlloads to deserialize yaml data from a string
• Update tests

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.10% ⚠️

Comparison is base (ec69e68) 97.31% compared to head (f190f39) 97.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3289      +/-   ##
==========================================
- Coverage   97.31%   97.21%   -0.10%     
==========================================
  Files         227      227              
  Lines       45464    45466       +2     
==========================================
- Hits        44242    44199      -43     
- Misses       1222     1267      +45     

Flag	Coverage Δ
linux	97.21% <100.00%> (+<0.01%)	⬆️
linux_replay	?

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Files Changed	Coverage Δ
synapse/telepath.py	93.66% <ø> (ø)
synapse/common.py	98.31% <100.00%> (+0.19%)	⬆️
synapse/lib/config.py	96.92% <100.00%> (ø)
synapse/lib/stormlib/yaml.py	100.00% <100.00%> (ø)
synapse/tools/moduser.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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