sstable: convert RawWriter into an interface

[jbowens]

Convert RawWriter into an interface that's satisfied by the existing RawRowWriter type. Future work will introduce a RawColWriter that writes sstables with columnar blocks.

[cockroach-teamcity]

This change is 

[RaduBerinde]

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

---

sstable/writer.go line 247 at r1 (raw file):

	Error() error
	// IsStrictObsolete returns true if the writer is configured to write and
	// enforce a 'strict obsolete' sstable.

[nit] What does "enforce" mean here? It used to mean that it would disallow Add() but that's not even part of the interface..

---

sstable/writer.go line 260 at r1 (raw file):

	// responsibility of the caller. S1 is solely the responsibility of the
	// callee.
	AddWithForceObsolete(

[nit] add that forceObsolete should always be false if !IsStrictObsolete()

[jbowens]

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

---

sstable/writer.go line 247 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] What does "enforce" mean here? It used to mean that it would disallow Add() but that's not even part of the interface..

Expanded a bit and plugged a reference to the 'strict obsolete' discussion in format.go

---

sstable/writer.go line 260 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] add that forceObsolete should always be false if !IsStrictObsolete()

If !StrictObsolete(), forceObsolete may be true but it's not required to be true where a IsStrictObsolete() file requires it. I added a comment clarifying that it's optional for not strict obsolete tables.

[jbowens]

TFTR!