graph: Round BigInt cache weight up to full bytes

Based on PR by https://github.com/cuiweixie

Author: lutter

graph/src/util/cache_weight.rs

@@ -140,7 +140,7 @@ impl CacheWeight for BigDecimal {
 
 impl CacheWeight for BigInt {
     fn indirect_weight(&self) -> usize {
-        self.bits() / 8
+        self.bits().div_ceil(8)
     }
 }