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/structs.proto

@@ -42,7 +42,8 @@ message FileInfoTruncated {
     int32                 block_size     = 13 [(gogoproto.customname) = "RawBlockSize"];
 
     // see bep.proto
-    uint32 local_flags = 1000;
+    uint32 local_flags  = 1000;
+    bytes  version_hash = 1001;
 
     bool deleted        = 6;
     bool invalid        = 7 [(gogoproto.customname) = "RawInvalid"];
@@ -54,9 +55,11 @@ message BlockList {
     repeated protocol.BlockInfo Blocks = 1 [(gogoproto.nullable) = false];
 }
 
-// BlocksHashOnly is used to only unmarshal the block list hash from a FileInfo
-message BlocksHashOnly {
-    bytes blocks_hash = 18;
+// IndirectionHashesOnly is used to only unmarshal the indirection hashes
+// from a FileInfo
+message IndirectionHashesOnly {
+    bytes blocks_hash  = 18;
+    bytes version_hash = 1001;
 }
 
 // For each folder and device we keep one of these to track the current