Merge bitcoin/bitcoin#27607: index: make startup more efficient

ca91c244ef1ba7eac6643d66a5fc56d3a2a8b550 index: verify blocks data existence only once (furszy)
fcbdaeef4d5a63e3e5b479c6fcad730eb86fb923 init: don't start indexes sync thread prematurely (furszy)
2ec89f1970935d27631bcd17b7923a79cdb1edbb refactor: simplify pruning violation check (furszy)
c82ef91eae38f385d408b095ebbc8a180ce52ebb make GetFirstStoredBlock assert that 'start_block' always has data (furszy)
430e7027a18870a296abb0bbd9332cbe40d8fdc0 refactor: index, decouple 'Init' from 'Start' (furszy)
225e213110602b4fd1d345167f5f92d26557f6c1 refactor: init indexes, decouple 'Start()' from the creation step (furszy)
2ebc7e68cc9d347807b646f601b27940c9590c89 doc: describe 'init load' thread actions (Martin Zumsande)
04575106b2529f495ce8110ddf7ed2247d4bc339 scripted-diff: rename 'loadblk' thread name to 'initload' (furszy)
ed4462cc78afd2065bbf5bd79728852b65b9ad7f init: start indexes sync earlier (furszy)

Pull request description:

  Simplifies index startup code, eliminating the `g_indexes_ready_to_sync` variable,
  deduplicating code and moving the prune violation check out of the `BaseIndex` class.

  Also makes startup more efficient by running the prune violation check once for all indexes
  instead of once for each index, and by delaying the prune violation check and moving it off
  of the main thread so the node can start up faster and perform the block data availability
  verification even when the '-reindex" or the "-reindex-chainstate" flags are enabled (which
  hasn't  being possible so far).

ACKs for top commit:
  ryanofsky:
    Code review ACK ca91c244ef1ba7eac6643d66a5fc56d3a2a8b550. Just rebase and suggested changes since last review (Start return check, and code simplification)
  TheCharlatan:
    re-ACK ca91c244ef1ba7eac6643d66a5fc56d3a2a8b550

Tree-SHA512: e9c98ce89aeb29e8d0f505f17b34aa54fe44efefbf017f4746e3b446ab4de25ade4f707254a0bbe4b99b69731b04a4067ce529eb7aa834ced196784b694cf7ce

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