Use read only transaction more on search

[lferran]

Description

Even if we had the shared transaction logic in place, many code in the search endpoints was creating extra transactions.
This caused a drop in performance specially when using the PG driver for the maindb (Hybrid on-prem deployments!)

The improvement on the performance tests is around 2x:

On main

Metrics summary:
- find_latency_s -> p50: 153.69ms p95: 283.02ms
- search_latency_s -> p50: 873.75ms p95: 1.19s
- suggest_latency_s -> p50: 249.35ms p95: 496.69ms
**** Molotov v2.6. Happy breaking! ****
SUCCESSES: 652 | FAILURES: 0
*** Bye ***

On the current branch

Metrics summary:
- find_latency_s -> p50: 58.67ms p95: 106.33ms
- search_latency_s -> p50: 443.13ms p95: 562.02ms
- suggest_latency_s -> p50: 118.93ms p95: 224.92ms
**** Molotov v2.6. Happy breaking! ****
SUCCESSES: 1276 | FAILURES: 0
*** Bye ***

How was this PR tested?

Describe how you tested this PR.

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (65c7d4f) 82.16% compared to head (ea83a68) 82.19%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
nucliadb/nucliadb/common/datamanagers/kb.py	72.41%	8 Missing ⚠️
nucliadb/nucliadb/common/maindb/pg.py	0.00%	3 Missing ⚠️
nucliadb/nucliadb/search/predict.py	50.00%	2 Missing ⚠️
nucliadb/nucliadb/common/datamanagers/cluster.py	90.90%	1 Missing ⚠️
nucliadb/nucliadb/ingest/orm/knowledgebox.py	50.00%	1 Missing ⚠️
nucliadb_utils/nucliadb_utils/keys.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1726      +/-   ##
==========================================
+ Coverage   82.16%   82.19%   +0.02%     
==========================================
  Files         335      336       +1     
  Lines       19803    19847      +44     
==========================================
+ Hits        16271    16313      +42     
- Misses       3532     3534       +2     

Flag	Coverage Δ
ingest	69.04% <50.00%> (-0.02%)	⬇️
node-sidecar	95.39% <ø> (ø)
nucliadb	70.34% <76.56%> (-0.06%)	⬇️
reader	79.82% <ø> (ø)
sdk	40.64% <ø> (ø)
standalone	88.29% <ø> (ø)
train	63.60% <ø> (ø)
utils	81.62% <0.00%> (+0.19%)	⬆️
writer	85.11% <ø> (+0.68%)	⬆️

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[lferran]

Related ticket [sc-8127]

[shortcut-integration]

This pull request has been linked to Shortcut Story #8127: refactor transaction context var on search.