fix: use ArcSwap for ChainStore::heaviest_tipset

[hanabi1224]

Summary of changes

# Command
oha -n 100000 -c 10 -m POST http://127.0.0.1:3456/rpc/v1 -H 'Content-Type: application/json' -D eth_getBlockByNumber.json

# payload
{
        "method": "eth_getBlockByNumber",
        "params": [
                "pending", false
        ],
        "id": 1,
        "jsonrpc": "2.0"
}

# PR
  Success rate: 100.00%   30s     40s     50s     60s      ││0.0004  0.0067  0.0130  0.0192  0.0255  0.0318  0.0380    │
  Total:────────14974.2865 ms──────────────────────────────┘└──────────────────────────────────────────────────────────┘
  Slowest:      38.0240 ms
  Fastest:      0.4130 ms
  Average:      1.4918 ms
  Requests/sec: 6678.1145

  Total data:   262.45 MiB
  Size/request: 2.69 KiB
  Size/sec:     17.53 MiB

Response time histogram:
   0.413 ms [1]     |
   4.174 ms [97631] |■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
   7.935 ms [1547]  |
  11.696 ms [290]   |
  15.457 ms [277]   |
  19.219 ms [97]    |
  22.980 ms [74]    |
  26.741 ms [48]    |
  30.502 ms [15]    |
  34.263 ms [8]     |
  38.024 ms [12]    |

Response time distribution:
  10.00% in 0.7832 ms
  25.00% in 0.9323 ms
  50.00% in 1.1811 ms
  75.00% in 1.6020 ms
  90.00% in 2.1580 ms
  95.00% in 2.8198 ms
  99.00% in 6.9136 ms
  99.90% in 21.3574 ms
  99.99% in 34.4245 ms
  
# main  
  Success rate: 100.00%   30s     40s     50s     60s      ││0.0006  0.0119  0.0231  0.0343  0.0455  0.0568  0.0680    │
  Total:────────23066.8244 ms──────────────────────────────┘└──────────────────────────────────────────────────────────┘
  Slowest:      67.9836 ms
  Fastest:      0.6298 ms
  Average:      2.2964 ms
  Requests/sec: 4335.2305

  Total data:   262.45 MiB
  Size/request: 2.69 KiB
  Size/sec:     11.38 MiB

Response time histogram:
   0.630 ms [1]     |
   7.365 ms [97930] |■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
  14.101 ms [921]   |
  20.836 ms [592]   |
  27.571 ms [182]   |
  34.307 ms [63]    |
  41.042 ms [13]    |
  47.777 ms [229]   |
  54.513 ms [22]    |
  61.248 ms [26]    |
  67.984 ms [21]    |

Response time distribution:
  10.00% in 1.2046 ms
  25.00% in 1.4188 ms
  50.00% in 1.7352 ms
  75.00% in 2.2705 ms
  90.00% in 3.0420 ms
  95.00% in 3.9198 ms
  99.00% in 15.4561 ms
  99.90% in 44.8020 ms
  99.99% in 66.3232 ms

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.36%. Comparing base (ae501fb) to head (05bb523).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/chain/store/chain_store.rs	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/chain/store/chain_store.rs	70.12% <80.00%> (-0.03%)	⬇️

... and 9 files with indirect coverage changes

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