[WIP] sstable: enable compression levels

[EdwardX29]

Enable compression levels by changing block.Compression to be composed of a Family and Level. Expose new enums to Cockroach.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewable status: 0 of 19 files reviewed, 2 unresolved discussions (waiting on @jbowens)

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sstable/block/compression.go line 86 at r1 (raw file):

var DefaultCompression = Compression{Family: DefaultCompressionFamily, Level: LevelDefault}
var NoCompression = Compression{Family: NoCompressionFamily, Level: LevelDefault}
var SnappyCompression = Compression{Family: SnappyCompressionFamily, Level: LevelDefault}

We should use ZstdLevelDefault, etc.

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options.go line 54 at r1 (raw file):

	// Zstd compression levels.
	ZstdCompressionLevel1  = block.Compression{Family: block.ZstdCompressionFamily, Level: block.ZstdLevel1}

I don't think level should be an enum.. It's always integers.