Add indirection for large version vectors. (#6376)

This adds indirection of large version vectors in the same manner as we
already to block lists. The effect is the same: less duplicated data in
some situations.

To mitigate the impact for when this indirection
wouldn't be needed I've added an indirection cutoff for both blocks and
the new version vector stuff: we don't do the indirection at all for
small block lists or small version vectors, instead storing it directly
like we used to do. This is faster for small files and small setups.

lib/db/keyer.go

@@ -65,6 +65,9 @@ const (
 
 	// KeyTypeBlockListMap <int32 folder ID> <block list hash> <file name> = <nothing>
 	KeyTypeBlockListMap = 14
+
+	// KeyTypeVersion <version hash> = Vector
+	KeyTypeVersion = 15
 )
 
 type keyer interface {
@@ -104,6 +107,9 @@ type keyer interface {
 
 	// Block lists
 	GenerateBlockListKey(key []byte, hash []byte) blockListKey
+
+	// Version vectors
+	GenerateVersionKey(key []byte, hash []byte) versionKey
 }
 
 // defaultKeyer implements our key scheme. It needs folder and device
@@ -328,7 +334,20 @@ func (k defaultKeyer) GenerateBlockListKey(key []byte, hash []byte) blockListKey
 	return key
 }
 
-func (k blockListKey) BlocksHash() []byte {
+func (k blockListKey) Hash() []byte {
+	return k[keyPrefixLen:]
+}
+
+type versionKey []byte
+
+func (k defaultKeyer) GenerateVersionKey(key []byte, hash []byte) versionKey {
+	key = resize(key, keyPrefixLen+len(hash))
+	key[0] = KeyTypeVersion
+	copy(key[keyPrefixLen:], hash)
+	return key
+}
+
+func (k versionKey) Hash() []byte {
 	return k[keyPrefixLen:]
 }