sstable: refactor ReaderOptions

[jbowens]

Add block.ReaderOptions and refactor sstable.ReaderOptions to contain them, and the blob reader to also use block.Reader and block.ReaderOptions.

[cockroach-teamcity]

This change is 

[jbowens]

Reviewable status: 0 of 12 files reviewed, 1 unresolved discussion (waiting on @sumeerbhola)

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sstable/options.go line 120 at r1 (raw file):

	// internal options can only be used from within the pebble package.
	internal sstableinternal.ReaderOptions

I removed this internal field, but I think it's alright to export the CacheOpts field since the referenced struct itself defined in the internal package.

[RaduBerinde]

Reviewable status: 0 of 12 files reviewed, 2 unresolved discussions (waiting on @jbowens and @sumeerbhola)

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sstable/options.go line 116 at r1 (raw file):

// this method is public, a caller outside the pebble package can't construct a
// value to pass to it.
func (o *ReaderOptions) SetInternalCacheOpts(cacheOpts sstableinternal.CacheOptions) {

[nit] We no longer need this method (we can just set CacheOpts directly now)

[jbowens]

TFTR!

Reviewable status: 0 of 19 files reviewed, 1 unresolved discussion (waiting on @sumeerbhola)

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sstable/options.go line 116 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] We no longer need this method (we can just set CacheOpts directly now)

Done