feat(node-sdk): decrease memory usage of rate limiter slightly

[roncohen]

Examples:

- 4cfdc69d4aa5521884b5e2558f9d7018b51b8bfa
+ TP3GnUqlUhiEteJVj51wGLUbi/o=

- 4e42ddd9f3c2e5232626829c2208936a985736e3
+ TkLd2fPC5SMmJoKcIgiTaphXNuM=

I tested the performance impact of this change in a microbenchmark and saw no significant change:

     name                                 hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · hashObject with hex encoding     406.29  2.1929  2.9307  2.4613  2.5955  2.9214  2.9215  2.9307  ±1.02%      204   fastest
   · hashObject with base64 encoding  398.01  2.2055  4.4220  2.5125  2.6363  3.0233  3.1191  4.4220  ±1.32%      200

 BENCH  Summary

  hashObject with hex encoding - test/hashobj.bench.ts > 
    1.02x faster than hashObject with base64 encoding