Fixed index out of range issue when loading large snapshot

[sgosiaco]

I recently found an issue when trying to load a snapshot created with a large number of rows (in my case 10 million).
It would result in an index out of range panic. I was able to reproduce this in the unit tests with around 3 million players.

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestLargeSnapshot$ github.com/kelindar/column

--- FAIL: TestLargeSnapshot (4.20s)
panic: runtime error: index out of range [128] with length 128 [recovered]
	panic: runtime error: index out of range [128] with length 128

goroutine 4 [running]:
testing.tRunner.func1.2({0x1053a25c0, 0x14059d74930})
	/usr/local/go/src/testing/testing.go:1389 +0x1c8
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1392 +0x384
panic({0x1053a25c0, 0x14059d74930})
	/usr/local/go/src/runtime/panic.go:838 +0x204
github.com/kelindar/column.(*Collection).readState.func1.1(0x14010b50090)
	/Users/sgosiaco/repos/column/snapshot.go:188 +0x274
github.com/kelindar/column.(*Collection).Query(0x1400012c370, 0x1400005bd18)
	/Users/sgosiaco/repos/column/collection.go:354 +0x48
github.com/kelindar/column.(*Collection).readState.func1(0x1400005bd88?, 0x105250968?)
	/Users/sgosiaco/repos/column/snapshot.go:184 +0x78
github.com/kelindar/iostream.(*Reader).ReadRange(0x1402a07f020, 0x1400010fe10)
	/Users/sgosiaco/go/pkg/mod/github.com/kelindar/iostream@v1.3.0/reader.go:227 +0x8c
github.com/kelindar/column.(*Collection).readState(0x1400012c370, {0x1053c3380?, 0x14000163900?})
	/Users/sgosiaco/repos/column/snapshot.go:183 +0x1d4
github.com/kelindar/column.(*Collection).Restore(0x1400012c370, {0x1053c3080, 0x140001014a0})
	/Users/sgosiaco/repos/column/snapshot.go:44 +0x54
github.com/kelindar/column.TestLargeSnapshot(0x140001151e0)
	/Users/sgosiaco/repos/column/snapshot_test.go:229 +0x174
testing.tRunner(0x140001151e0, 0x1053bf558)
	/usr/local/go/src/testing/testing.go:1439 +0x110
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1486 +0x300
FAIL	github.com/kelindar/column	4.339s
FAIL

Looking at the code I saw that currently the maximum number of chunks able to be read is 128 due to the initialized size of the commits array. I've changed this to a map to allow for an unknown number of chunks/commits to be loaded since when saving the snapshot there doesn't seem to be a limit on the potential number of chunks that can be saved.