init: don't start indexes sync thread prematurely
4708298172e0
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Description

init: don't start indexes sync thread prematurely

Summary:
By moving the 'StartIndexes()' call into the 'initload'
thread, we can remove the threads active wait. Optimizing
the available resources.

The only difference with the current state is that now the
indexes threads will only be started when they can process
work and not before it.

Depends on D17748

Notes:

we diverge from the source material in init.cpp because of missing backports (core#27861 and ancestors) that are intentionally skipped because they depend on the massive multiprocessing project. This affects a single line, and a TODO has been added to explain the divergence.
the test changes in feature_index_prune.py are applicable in the next commit, in this commit they would break the tests

Test Plan: ninja all check check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien