[chronik] Split indexer setup into prepare_paths and prepare_db

Summary:
This makes the code easier to read, but most important it makes the tests capable to execute without requiring a call to setup().
A follow up diff will add the node bridge to the indexer, requiring cxx which is not linked to the test profile (and I have no solution to achieve that as it's not supported by corrosion).

The db wiping unit test has been removed as it is already fully covered by the chronik_resync.py functional test.

Test Plan:

ninja all check-crate-chronik

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck, alitayin

Differential Revision: https://reviews.bitcoinabc.org/D18428