sstable/blob: new package

[jbowens]

Add blob FileWriter type for writing out blob files, consisting of a series of
value blocks, a value block index and a footer.

Add the initial, largely unimplemented stencil of a FileReader type.

Informs #112.

[cockroach-teamcity]

This change is 

[sumeerbhola]

flushing some comments on the first commit.

Reviewed 3 of 3 files at r1.
Reviewable status: 2 of 7 files reviewed, 5 unresolved discussions (waiting on @jbowens)

---

sstable/block/compression.go line 357 at r1 (raw file):

// CompressAndChecksum compresses and checksums the block data, returning a
// PhysicalBuffer that is owned by the caller.  If non-nil, the returned

should this say PhysicalBlock instead of PhysicalBuffer?
I am guessing that the slice in PhysicalBlock is backed by BufHandle but it isn't explicit in this code comment.

---

sstable/block/compression.go line 377 at r1 (raw file):

	// Use the compressedBuf as the new b.h.
	pbHandle := b.h
	b.h = compressedBuf

This is a bit different from what we used to do in valblk.Writer.compressAndFlush in that given the difference in sizes, it was trying to use the compressed and uncompressed *blockBuffers separate, and not mix the pools.
Perhaps that wasn't necessary, but I am curious about the motivation.

---

sstable/block/compression.go line 404 at r1 (raw file):

// Release releases the buffer back to the pool for reuse.
func (h *BufHandle) Release() {
	if invariants.Enabled && (h.pool == nil) != (h.b == nil) {

this suggests we have an invariant h != nil, so the next if predicate shouldn't need to do h != nil.

---

sstable/block/compression.go line 410 at r1 (raw file):

	// maximum to the pool. This avoids holding on to occassional large buffers
	// necesary for, for example, single large values.
	if h != nil && h.b != nil && (h.pool.Max == 0 || len(h.b) < h.pool.Max) {

is there a reason to allow Max == 0 to mean unlimited, given we don't use it?

---

sstable/block/compression.go line 450 at r1 (raw file):

		return v.(*BufHandle)
	}
	return &BufHandle{b: make([]byte, 0, max(p.Default, 1024)), pool: p}

Do we really need this max in that we are only creating two of these bufferSyncPools with reasonable defaults. And if someone uses a smaller default, they potentially have a good reason for it.