Store total key-value size in table footer

[jarifibrahim]

This PR stores the total key-value size in a table in the table footer. The key-value size can be accessed via db.Tables(..) call. It returns the list of all tables along with the total size of key-values.

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This change is 

[coveralls]

Coverage decreased (-0.1%) to 77.414% when pulling a243b6c on ibrahim/table-kv-size into f5b6321 on master.

[coveralls]

Coverage decreased (-7.6%) to 69.939% when pulling 4130bba on ibrahim/table-kv-size into f5b6321 on master.

[manishrjain]

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami and @jarifibrahim)

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table/builder.go, line 132 at r1 (raw file):

	v.EncodeTo(b.buf)
	b.tableIndex.KvSize += uint64(uint32(headerSize) + uint32(len(diffKey)) + v.EncodedSize())

EstimatedSize

• Size of KV on SST
• Size of KV on Value Log
• Sum these two.
• Do not consider compression / encryption.

Or, perhaps do consider compression. Use the compressed size of the block and add the value log sizes to it.

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table/table.go, line 445 at r1 (raw file):

// TotalKVSize returns the total size of key-values stored in this table.
func (t *Table) TotalKVSize() uint64 { return t.kvSize }

EstimatedSize

[manishrjain]

Reviewed 11 of 11 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami and @jarifibrahim)