colblk: cache KeyspanDecoder in block metadata

[RaduBerinde]

No description provided.

[cockroach-teamcity]

This change is 

[jbowens]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @RaduBerinde)

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sstable/colblk/data_block.go line 774 at r1 (raw file):

	defer func() {
		if r := recover(); r != nil {
			err = base.CorruptionErrorf("error initializing data block metadata: %v", r)

nit: should we say "index block metadata"

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sstable/colblk/data_block.go line 794 at r1 (raw file):

	defer func() {
		if r := recover(); r != nil {
			err = base.CorruptionErrorf("error initializing data block metadata: %v", r)

and "keyspan block metadata"

[RaduBerinde]

TFTR!

Reviewable status: 2 of 4 files reviewed, 1 unresolved discussion

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sstable/colblk/data_block.go line 794 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

and "keyspan block metadata"

Done.