go/neo/proto: Update 'Compression' to int to support different compression algorithms

With nexedi/neoppod@fd80cc30 NEO/py added support to encode the compression
algorithm with the 'Compression' parameter. Before this, compression could
only be true (= with compression) or false (= without compression). Now
the absence of compression is encoded with 0. Any other number than 0
encodes a compression algorithm. The mapping is currently:

	1 = zlib

In the future, 2 could mean zstd [1].

[1] https://github.com/facebook/zstd/issues/1134

/reviewed-by @kirr
/reviewed-on !6

go/neo/proto/proto.go

@@ -332,6 +332,11 @@ func (a *Address) neoDecodeN(b []byte) (uint64, bool) {
 // Checksum is a SHA1 hash.
 type Checksum [20]byte
 
+// Compression is an integer that tells the compression algorithm:
+//	0 = no compression algorithm is used
+//	1 = zlib is used
+type Compression uint8
+
 // PTid is Partition Table identifier.
 //
 // Zero value means "invalid id" (<-> None in py.PPTID)
@@ -707,7 +712,7 @@ type AnswerRebaseObject struct {
 	Serial         zodb.Tid
 	ConflictSerial zodb.Tid
 	// FIXME POption('data')
-		Compression bool
+		Compression Compression
 		Checksum    Checksum
 		Data        *mem.Buf
 }
@@ -721,7 +726,7 @@ type AnswerRebaseObject struct {
 type StoreObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        []byte // TODO -> msg.Buf, separately (for writev)
 	DataSerial  zodb.Tid
@@ -776,7 +781,7 @@ type AnswerObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
 	NextSerial  zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        *mem.Buf // TODO encode -> separately (for writev)
 	DataSerial  zodb.Tid
@@ -1211,7 +1216,7 @@ type AddTransaction struct {
 type AddObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        *mem.Buf
 	DataSerial  zodb.Tid

go/neo/proto/proto_test.go

@@ -182,7 +182,7 @@ func TestMsgMarshal(t *testing.T) {
 		{&StoreObject{
 			Oid:         0x0102030405060708,
 			Serial:      0x0a0b0c0d0e0f0102,
-			Compression: false,
+			Compression: 0,
 			Checksum:    Checksum{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}, // XXX simpler?
 			Data:        []byte("hello world"),
 			DataSerial:  0x0a0b0c0d0e0f0103,
@@ -198,7 +198,7 @@ func TestMsgMarshal(t *testing.T) {
 			hex("97") +
 			hex("c408") + hex("0102030405060708") +
 			hex("c408") + hex("0a0b0c0d0e0f0102") +
-			hex("c2") +
+			hex("00") +
 			hex("c414") + hex("0102030405060708090a0b0c0d0e0f1011121314") +
 			hex("c40b") + "hello world" +
 			hex("c408") + hex("0a0b0c0d0e0f0103") +