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/protocol/bep.proto

@@ -119,6 +119,9 @@ message FileInfo {
     // received (we make sure to zero it), nonetheless we need it on our
     // struct and to be able to serialize it to/from the database.
     uint32 local_flags = 1000;
+    // The version_hash is an implementation detail and not part of the wire
+    // format.
+    bytes version_hash = 1001;
 
     bool deleted        = 6;
     bool invalid        = 7 [(gogoproto.customname) = "RawInvalid"];