Fixed index out of range issue when loading large snapshot (#88)

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/[email protected]/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.

snapshot.go

@@ -163,9 +163,9 @@ func (c *Collection) writeState(dst io.Writer) (int64, error) {
 
 // readState reads a collection snapshotted state from the underlying reader. It
 // returns the last commit IDs for each chunk.
-func (c *Collection) readState(src io.Reader) ([]uint64, error) {
+func (c *Collection) readState(src io.Reader) (map[commit.Chunk]uint64, error) {
 	r := iostream.NewReader(src)
-	commits := make([]uint64, 128)
+	commits := make(map[commit.Chunk]uint64)
 
 	// Read the version and make sure it matches
 	version, err := r.ReadUvarint()
@@ -185,7 +185,7 @@ func (c *Collection) readState(src io.Reader) ([]uint64, error) {
 			txn.dirty.Set(uint32(chunk))
 
 			// Read the last written commit ID for the chunk
-			if commits[chunk], err = r.ReadUvarint(); err != nil {
+			if commits[commit.Chunk(chunk)], err = r.ReadUvarint(); err != nil {
 				return err
 			}
 

snapshot_test.go

@@ -202,6 +202,34 @@ func TestSnapshot(t *testing.T) {
 	assert.Equal(t, amount, output.Count())
 }
 
+func TestLargeSnapshot(t *testing.T) {
+	amount := 3_000_000
+	buffer := bytes.NewBuffer(nil)
+	input := loadPlayers(amount)
+
+	var wg sync.WaitGroup
+	wg.Add(amount)
+	go func() {
+		for i := 0; i < amount; i++ {
+			assert.NoError(t, input.QueryAt(uint32(i), func(r Row) error {
+				r.SetString("name", "Roman")
+				return nil
+			}))
+			wg.Done()
+		}
+	}()
+
+	// Start snapshotting
+	assert.NoError(t, input.Snapshot(buffer))
+	assert.NotZero(t, buffer.Len())
+
+	// Restore the snapshot
+	wg.Wait()
+	output := newEmpty(amount)
+	assert.NoError(t, output.Restore(buffer))
+	assert.Equal(t, amount, output.Count())
+}
+
 func TestSnapshotFailures(t *testing.T) {
 	input := NewCollection()
 	input.CreateColumn("name", ForString())