02b9511d6b tests: add tests for GetCoinsCacheSizeState (James O'Beirne)
b17e91d842 refactoring: introduce CChainState::GetCoinsCacheSizeState (James O'Beirne)
Pull request description:
This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):
Parent PR: #15606
Issue: #15605
Specification: https://github.com/jamesob/assumeutxo-docs/tree/master/proposal
---
This pulls out the routine for detection of how full the coins cache is from
FlushStateToDisk. We use this logic independently when deciding when to flush
the coins cache during UTXO snapshot activation ([see here](231fb5f17e (diff-24efdb00bfbe56b140fb006b562cc70bR5275))).
ACKs for top commit:
ariard:
Code review ACK 02b9511.
ryanofsky:
Code review ACK 02b9511d6b. Just rebase, new COIN_SIZE comment, and new test message since last review
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