wallet: add a rescan_bc command and rescan_blockchain RPC

Blockchain hashes and key images are flushed, and blocks are
pulled anew from the daemon.
The console command is shortened to match bc_height.
This should make it a lot easier on users who are currently
told to remove this particular cache file but keep the keys
one, etc, etc.

src/wallet/wallet_rpc_server_commands_defs.h

@@ -391,6 +391,21 @@ namespace wallet_rpc
     };
   };
 
+  struct COMMAND_RPC_RESCAN_BLOCKCHAIN
+  {
+    struct request
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+
+    struct response
+    {
+      BEGIN_KV_SERIALIZE_MAP()
+      END_KV_SERIALIZE_MAP()
+    };
+  };
+
 }
 }