perf(cartridge/execute_sql): use 'callbre' instead of 'callro' to execute SQL

Basically callbre means that we call replicaset with two additional parameters:
    1) prefer_replica = true. We can use read from replicas because
        transactional consistency is out of Sbroad scope. Basically we
        do not care about data consistency at all.
    2) balance = true. This parameters enables internal vshard load
        balancing between replicaset nodes in round-robin fashion,
        with a preference for replicas if prefer_replica=true is also set

src/executor/engine/cartridge.rs

@@ -271,7 +271,7 @@ impl Runtime {
         local futures = {}
 
         for _, replica in pairs(replicas) do
-             local future, err = replica:callro("box.execute", { query }, {is_async = true})
+             local future, err = replica:callbre("box.execute", { query }, {is_async = true})
              if err ~= nil then
                  error(err)
              end