Update rfc for the livestate feature

[ffjlabo]

What this PR does:

as title

Why we need it:

Which issue(s) this PR fixes:

Part of #5363

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.22%. Comparing base (511ee8f) to head (aaeb9ee).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5448      +/-   ##
==========================================
+ Coverage   26.09%   26.22%   +0.13%     
==========================================
  Files         451      452       +1     
  Lines       48824    48872      +48     
==========================================
+ Hits        12741    12819      +78     
+ Misses      35065    35027      -38     
- Partials     1018     1026       +8     

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[Warashi]

LGTM