validation: don't erase coins cache on prune flushes
Summary:
This concludes backport of core#28280
https://github.com/bitcoin/bitcoin/pull/28280/commits/589db872e116779ab9cae693171ac8a8c02d9923
Depends on D18620
Test Plan:
$ src/bitcoind -datadir=/data0/ecashd_test_pruning_ibd/ -dbcache=16384 -prune=550 2025-09-14T19:53:14Z Bitcoin ABC version v0.31.12-a36e609718a2 (release build) ... 2025-09-14T21:37:21Z Leaving InitialBlockDownload (latching to false) ... 2025-09-14T21:37:48Z UpdateTip: new best=000000000000000015e71b7a821a81f8f35ed3bf946c20ec1cb7722925e07dec height=914517 version=0x2e000000 log2_work=88.533540 tx=299825555 date='2025-09-14T21:37:41Z' progress=1.000000 cache=6914.2MiB(39061252txo)
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18621