snapshot: fix race in abort/generate

[darioush]

Excerpts from https://github.com/ava-labs/subnet-evm/actions/runs/8725583610/job/23938886724?pr=1154

Previous write at 0x00c0075f5590 by goroutine 14701:
  github.com/ava-labs/subnet-evm/core/state/snapshot.(*diskLayer).generate()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/generate.go:402 +0x238e
  github.com/ava-labs/subnet-evm/core/state/snapshot.generateSnapshot.func1()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/generate.go:160 +0x44

Goroutine 14701 (running) created at:
  github.com/ava-labs/subnet-evm/core/state/snapshot.generateSnapshot()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/generate.go:160 +0x71b
  github.com/ava-labs/subnet-evm/core/state/snapshot.(*Tree).Rebuild()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/snapshot.go:784 +0x4a6
  github.com/ava-labs/subnet-evm/core/state/snapshot.New()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/snapshot.go:227 +0x571
  github.com/ava-labs/subnet-evm/core.(*BlockChain).initSnapshot()
      /home/runner/work/subnet-evm/subnet-evm/core/blockchain.go:1880 +0x6bc
  github.com/ava-labs/subnet-evm/core.(*BlockChain).reprocessState()
      /home/runner/work/subnet-evm/subnet-evm/core/blockchain.go:1982 +0x1564
  github.com/ava-labs/subnet-evm/core.(*BlockChain).loadLastState()

Read at 0x00c0075f5590 by goroutine 14527:
  github.com/ava-labs/subnet-evm/core/state/snapshot.(*diskLayer).abortGeneration()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/snapshot.go:580 +0x18d
  github.com/ava-labs/subnet-evm/core/state/snapshot.(*Tree).AbortGeneration()
      /home/runner/work/subnet-evm/subnet-evm/core/state/snapshot/snapshot.go:564 +0x97
  github.com/ava-labs/subnet-evm/core.(*BlockChain).flattenSnapshot()
      /home/runner/work/subnet-evm/subnet-evm/core/blockchain.go:5[94](https://github.com/ava-labs/subnet-evm/actions/runs/8725583610/job/23938886724?pr=1154#step:6:95) +0x88
  github.com/ava-labs/subnet-evm/core.(*BlockChain).reprocessState()
      /home/runner/work/subnet-evm/subnet-evm/core/blockchain.go:19[95](https://github.com/ava-labs/subnet-evm/actions/runs/8725583610/job/23938886724?pr=1154#step:6:96) +0x16d1
  github.com/ava-labs/subnet-evm/core.(*BlockChain).loadLastState()
      /home/runner/work/subnet-evm/subnet-evm/core/blockchain.go:816 +0xcf6
  github.com/ava-labs/subnet-evm/core.NewBlockChain()