Implement RHT.GC

[hackerwins]

What this PR does / why we need it:

Implement RHT.GC

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• [ ] Added relevant tests or not required
• [ ] Didn't break anything

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 17.50000% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 50.71%. Comparing base (24950f5) to head (940c06d).

Files	Patch %	Lines
pkg/document/crdt/tree.go	0.00%	15 Missing :warning:
pkg/document/crdt/rht.go	30.00%	14 Missing :warning:
pkg/document/change/context.go	0.00%	2 Missing :warning:
pkg/document/crdt/root.go	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #862 +/- ## ========================================== - Coverage 50.81% 50.71% -0.10% ========================================== Files 70 70 Lines 10285 10311 +26 ========================================== + Hits 5226 5229 +3 - Misses 4530 4553 +23 Partials 529 529 ```

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