Fixed two bugs in decompression sorted merge code

[jnidzwetzki]

SQLSmith found two bugs in the compression sorted merge code.

• The unused_batch_states are not initialized properly. Therefore, non-existing unused batch states can be part of the BMS. This patch fixes the initialization.

• For performance reasons, We reuse the same TupleDesc across all TupleTableSlots. PostgreSQL sometimes uses TupleDesc data structures with active reference counting. The way we use the TupleDesc structures collides with the reference counting of PostgreSQL. This patch introduces a private TupleDesc copy without reference counting.

Link to failed SQLSmith run: https://github.com/timescale/timescaledb/actions/runs/5233117820/jobs/9448359165

[codecov]

Codecov Report

Merging #5774 (48d8ce1) into main (4dce87a) will increase coverage by 7.47%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##             main    #5774      +/-   ##
==========================================
+ Coverage   80.39%   87.86%   +7.47%     
==========================================
  Files         239      239              
  Lines       48760    55478    +6718     
  Branches    12220    12287      +67     
==========================================
+ Hits        39199    48745    +9546     
- Misses       4187     4837     +650     
+ Partials     5374     1896    -3478     

Impacted Files	Coverage Δ
tsl/src/nodes/decompress_chunk/exec.c	92.94% <91.30%> (+11.74%)	⬆️

... and 222 files with indirect coverage changes

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[timescale-automation]

Automated backport to 2.11.x not done: cherry-pick failed.

Git status

HEAD detached at origin/2.11.x
You are currently cherry-picking commit 9c7ae3e8a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .unreleased/bugfix_5774
	modified:   tsl/src/nodes/decompress_chunk/exec.c
	modified:   tsl/test/expected/compression_sorted_merge-12.out
	modified:   tsl/test/expected/compression_sorted_merge-13.out
	modified:   tsl/test/expected/compression_sorted_merge-14.out
	modified:   tsl/test/expected/compression_sorted_merge-15.out
	modified:   tsl/test/sql/compression_sorted_merge.sql.in

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tsl/src/nodes/decompress_chunk/exec.h

Job log