Alpha/Delegation/Migration: reassign rolls

This patch is part of the migration code from `alpha_003` to `athens_004`.

It performs the roll size update to 8000 tokens.

src/proto_alpha/lib_protocol/src/init_storage.ml

@@ -45,6 +45,9 @@ let prepare_first_block ctxt ~typecheck ~level ~timestamp ~fitness =
       Vote_storage.freeze_listings ctxt >>=? fun ctxt ->
       return ctxt
   | Alpha_previous ->
+      (* Reallocated rolls to delegates, since rolls are now 8k tez. *)
+      Roll_storage.update_tokens_per_roll ctxt
+        Constants_repr.default.tokens_per_roll >>=? fun ctxt ->
       (* Update hard gas limits to the new defaults. *)
       Raw_context.patch_constants ctxt begin fun constants ->
         { constants with