Reuse buffer for decompression

[jarifibrahim]

This commit uses a slice pool to store buffers that can be reused for decompression.

Fixes #1239

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This change is 

[manishrjain]

Use slices and be careful about ownership.

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami and @jarifibrahim)

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table/table.go, line 80 at r1 (raw file):

var slicePool = sync.Pool{
	New: func() interface{} {
		b := make([]byte, 5<<10)

Use y.Slice.

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table/table.go, line 636 at r1 (raw file):

		return data, nil
	case options.Snappy:
		slicePool.Put(&data)

Use defer or something for this. Can't give up ownership before usage.

[jarifibrahim]

The code in this PR won't work. The []byte slice we're inserting into the sync pool is read from the mmaped sst file and any changes to the data byte slice leads to modification of the mmapped buffer of the sst.

The issue is similar to the issue seen in the following snippet.

func do(b []byte) {
	copy(b, []byte("d"))
}

func main() {
	b := []byte("foo")
	fmt.Println(string(b)) // foo
	do(b)
	fmt.Println(string(b)) // doo
}