fix(driver): compoundindexer should turn on is_merge

[hanxiao]

without is_merge=True, all scored info is lost due to copy override

[codecov]

Codecov Report

Merging #1888 (687c43e) into master (6716be2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1888   +/-   ##
=======================================
  Coverage   86.28%   86.28%           
=======================================
  Files         147      147           
  Lines        7015     7015           
=======================================
  Hits         6053     6053           
  Misses        962      962           

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[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1015, delta to last 3 avg.: -5%
• 🐢🐢 query QPS at 16, delta to last 3 avg.: -13%

Breakdown

Version	Index QPS	Query QPS
current	1015	16
0.9.29	1018	16
0.9.28	1121	20

Backed by latency-tracking. Further commits will update this comment.

[JoanFM]

@hanxiao @cristianmtr now I see why it was True by default, I think then we should set it back to True by default and find a fix for the problem of duplicated shapes.