Fix snapshot race

[kelindar]

This should fix the following race

WARNING: DATA RACE
Write at 0x00c00432b000 by goroutine [9](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:10)1:
  github.com/kelindar/bitmap.(*Bitmap).Set()
      /home/runner/go/pkg/mod/github.com/kelindar/bitmap@v1.4.1/bitmap.go:27 +0xb3
  github.com/kelindar/column.(*Collection).next()
      /home/runner/work/column/column/collection.go:94 +0x84
  github.com/kelindar/column.(*Txn).insert()
      /home/runner/work/column/column/txn.go:336 +0x64
  github.com/kelindar/column.(*Txn).Insert()
      /home/runner/work/column/column/txn.go:329 +0xe9
  github.com/kelindar/column.(*Collection).Insert.func1()
      /home/runner/work/column/column/collection.go:152 +0x58
  github.com/kelindar/column.(*Collection).Query()
      /home/runner/work/column/column/collection.go:293 +0x73
  github.com/kelindar/column.(*Collection).Insert()
      /home/runner/work/column/column/collection.go:151 +0x84
  github.com/kelindar/column.TestSnapshotFailures.func3()
      /home/runner/work/column/column/snapshot_test.go:209 +0x47
Previous read at 0x00c00432b000 by goroutine 188:
  github.com/kelindar/bitmap.Bitmap.Range()
      /home/runner/go/pkg/mod/github.com/kelindar/bitmap@v1.4.1/range.go:17 +0x5a
  github.com/kelindar/column.(*Collection).writeState.func1.1()
      /home/runner/work/column/column/snapshot.go:142 +0x1cd
  github.com/kelindar/column.(*Collection).readChunk()
      /home/runner/work/column/column/txn_lock.go:[10](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:11)9 +0x210
  github.com/kelindar/column.(*Collection).writeState.func1()
      /home/runner/work/column/column/snapshot.go:[13](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:14)2 +0x9a
  github.com/kelindar/iostream.(*Writer).WriteRange()
      /home/runner/go/pkg/mod/github.com/kelindar/iostream@v1.3.0/writer.go:245 +0x98
  github.com/kelindar/column.(*Collection).writeState()
      /home/runner/work/column/column/snapshot.go:131 +0x2dc
  github.com/kelindar/column.(*Collection).Snapshot()
      /home/runner/work/column/column/snapshot.go:68 +0x164
  github.com/kelindar/column.TestSnapshotFailures()
      /home/runner/work/column/column/snapshot_test.go:217 +0x1a6
  testing.tRunner()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:[14](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:15)39 +0x213
  testing.(*T).Run.func1()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1486 +0x47
Goroutine 91 (running) created at:
  github.com/kelindar/column.TestSnapshotFailures()
      /home/runner/work/column/column/snapshot_test.go:209 +0x13c
  testing.tRunner()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1439 +0x213
  testing.(*T).Run.func1()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1486 +0x47
Goroutine 188 (running) created at:
  testing.(*T).Run()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1486 +0x724
  testing.runTests.func1()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1839 +0x99
  testing.tRunner()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1439 +0x213
  testing.runTests()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:1837 +0x7e4
  testing.(*M).Run()
      /opt/hostedtoolcache/go/1.18.3/x64/src/testing/testing.go:[17](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:18)[19](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:20) +0xa71
  main.main()
      _testmain.go:[27](https://github.com/kelindar/column/runs/6852358984?check_suite_focus=true#step:5:28)5 +0x3a9