Optimize ZUNION[STORE] command by removing unnecessary accumulator dict

[RayaCoo]

In the past implementation of ZUNION and ZUNIONSTORE commands, we first create a temporary dict called accumulator. After adding all member-score mappings to accumulator, we still need to convert accumulator back to the final dict dstzset->dict. However, we can directly use dstzset->dict to avoid the additional copy operation.

This PR removes the accumulator dict and directly usesdstzset->dictto store the member-score mappings.

• Test
  First, I added 300 unique elements to two sorted sets called 'zunion_test1' and 'zunion_test2'. Then, I tested zunion and zunionstore on these two sorted sets. The test results shown below indicate that the performance of both zunion and zunionstore improved about 31%.

ZUNION

unstable

./valkey-benchmark -P 10 -n 100000 zunion 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 2713.41 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      146.252     3.464   153.343   182.015   184.959   192.895

This PR

./valkey-benchmark -P 10 -n 100000 zunion 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 3543.84 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      108.259     2.984   114.239   141.695   145.151   160.255

ZUNIONSTORE

unstable

./valkey-benchmark -P 10 -n 100000 zunionstore out 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 3168.07 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      157.511     3.368   183.167   189.311   193.535   231.679

This PR

./valkey-benchmark -P 10 -n 100000 zunionstore out 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 4144.73 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      120.374     2.648   141.823   149.119   153.855   183.167

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.27%. Comparing base (36e81d9) to head (325979a).
Report is 38 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #829      +/-   ##
============================================
- Coverage     70.46%   70.27%   -0.20%     
============================================
  Files           112      112              
  Lines         61135    61463     +328     
============================================
+ Hits          43077    43191     +114     
- Misses        18058    18272     +214     

Files	Coverage Δ
src/t_zset.c	95.60% <100.00%> (+0.04%)	⬆️

... and 26 files with indirect coverage changes