[FAB-1773] Add metadata field for orderer use

https://jira.hyperledger.org/browse/FAB-1773

A consensus implementation may need to read the ledger and extract
metadata from it during operation (most likely when booting up).

As an example, the Kafka-based orderer, should read the offset of the
last envelope it placed into a block *and* wrote to the local ledger, and
use that value to resume consumption of chain's partition.

This changeset:
1. Introduces a metadata field for the described use above
2. Sizes and populates the `Metadata` field of a newly-constructed block
appropriately.

These changes will be put into use in a follow-up changeset [FAB-1774].

Change-Id: Ib3d7d8f77c1a888ebec5790a144457913b201202
Signed-off-by: Kostas Christidis <kostas@christidis.io>

protos/common/block.go

@@ -28,9 +28,13 @@ func NewBlock(seqNum uint64, previousHash []byte) *Block {
 	block.Header.Number = seqNum
 	block.Header.PreviousHash = previousHash
 	block.Data = &BlockData{}
-	block.Metadata = &BlockMetadata{
-		Metadata: [][]byte{[]byte{}, []byte{}, []byte{}},
+
+	var metadataContents [][]byte
+	for i := 0; i < len(BlockMetadataIndex_name); i++ {
+		metadataContents = append(metadataContents, []byte{})
 	}
+	block.Metadata = &BlockMetadata{Metadata: metadataContents}
+
 	return block
 }
 

protos/common/common.proto

@@ -43,11 +43,13 @@ enum HeaderType {
     DELIVER_SEEK_INFO = 5;         // Used as the type for Envelope messages submitted to instruct the Deliver API to seek
 }
 
-// This enum enlist indexes of the block metadata array
+// This enum enlists indexes of the block metadata array
 enum BlockMetadataIndex {
     SIGNATURES = 0;             // Block metadata array position for block signatures
     LAST_CONFIGURATION = 1;     // Block metadata array poistion to store last configuration block sequence number
     TRANSACTIONS_FILTER = 2;    // Block metadata array poistion to store serialized bit array filter of invalid transactions
+    ORDERER = 3;                // Block metadata array position to store operational metadata for orderers
+                                // e.g. For Kafka, this is where we store the last offset written to the local ledger.
 }
 
 // LastConfiguration is the encoded value for the Metadata message which is encoded in the LAST_CONFIGURATION block metadata index