Docs deprecation warnings (SYN-4257)

[vEpiphyte]

• Fix use warnings.warn in synapse.common
• Add docs for enabling deprecation warnings.
• Fix use of List.length() in service.del implementation.
• Update agenda to create and use a view directly instead of calling core.eval.
• Generally remove Core.eval / CoreApi.eval from unit tests.
• Update function docstrings for deprecated functions to note their status.

[codecov]

Codecov Report

Base: 97.17% // Head: 97.17% // No change to project coverage 👍

Coverage data is based on head (7fbf8a9) compared to base (7fbf8a9).
Patch has no changes to coverable lines.

❗ Current head 7fbf8a9 differs from pull request most recent head de81266. Consider uploading reports for the commit de81266 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2905   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files         215      215           
  Lines       42084    42084           
=======================================
  Hits        40897    40897           
  Misses       1187     1187           

Flag	Coverage Δ
linux	97.07% <0.00%> (ø)
linux_replay	96.24% <0.00%> (ø)

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