[BREAKING] feat(index): Use flatbuffers instead of protobuf

[jarifibrahim]

This PR

• Uses flatbuffers instead of protobufs for table index and directly stores byte slices in the cache.
• Uses MaxVersion to pick the oldest tables for compaction first.
• Uses leveldb/bloom so that we can test it without unmarshal
• Adds uncompressed size and key count in table index.
• Updates write bench tool to use managed mode.

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This change is 

[manishrjain]

Reviewed 13 of 13 files at r1.
Reviewable status: all files reviewed, 7 unresolved discussions (waiting on @ashish-goswami and @jarifibrahim)

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flatbuffer.fbs, line 1 at r1 (raw file):

namespace fb;

add license.

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table/builder.go, line 78 at r1 (raw file):

// blockOffset is used to create the index in builder.Finish
type blockOffset struct {

Don't need this struct. Create blockoffsets via Flatbuffers, and lay them out on z.Calloc or something.

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table/builder.go, line 96 at r1 (raw file):

	entryOffsets  []uint32 // Offsets of entries present in current block.
	indexBuidler  *flatbuffers.Builder
	offsets       []*blockOffset

Don't use Go memory for this.

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table/builder.go, line 404 at r1 (raw file):

			off := b.offsets[i]
			// Length of the block is end minus the start.
			off.len = bl.end - bl.start

don't call it len. Len was ok, because of capital L. Maybe call it sz now.

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table/table.go, line 107 at r1 (raw file):

	var buf []byte
	for j := 0; j < ti.BloomFilterLength(); j++ {
		buf = append(buf, byte(ti.BloomFilter(j)))

this is weird code.

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table/table.go, line 125 at r1 (raw file):

// Key retuns the key for the blockoffset.
func (bo *fBlockOffset) key() []byte {

Why are you allocating memory in these functions? The point of FBs are to access the bytes directly off the marshaled structures.

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table/table.go, line 542 at r1 (raw file):

	if blk.data, err = t.read(blk.offset, int(ko.Len())); err != nil {
		return nil, errors.Wrapf(err,
			"failed to read from file: %s at offset: %d, len: %d", t.fd.Name(), blk.offset, ko.Len())

100 chars.

[CLAassistant]

All committers have signed the CLA.

[manishrjain]

Good to merge.

Reviewed 1 of 10 files at r2, 1 of 5 files at r5, 2 of 7 files at r6, 4 of 9 files at r7, 1 of 2 files at r9, 1 of 1 files at r10, 6 of 8 files at r11, 1 of 7 files at r13, 11 of 11 files at r14.
Reviewable status: 27 of 28 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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fb/gen.sh, line 5 at r4 (raw file):

set -e

# You will need the flatbuffer compiler (flatc) https://google.github.io/flatbuffers/flatbuffers_guide_building.html

100 chars.