graph: Round BigInt cache weight up to full bytes

Use ceiling division when converting bit length to bytes so values whose bit width is not a multiple of eight are not undercounted for cache accounting.

Author: cuiweixie

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() + 7) / 8
     }
 }