refactor: rename FlushStateMode::ALWAYS to FORCE_FLUSH

This prepares the addition of `FORCE_SYNC`.

`empty_cache` in `FlushStateToDisk` was moved up to be reusable and `FlushStateMode::FORCE_FLUSH` was used as a placeholder before we properly split the two new states.
`log_utxocache_flush.py` was regenerated and the alignment adjusted for the wider `FlushStateMode` values.

Co-authored-by: Pieter Wuille <pieter@wuille.net>
Co-authored-by: optout <13562139+optout21@users.noreply.github.com>

doc/tracing.md

@@ -185,8 +185,8 @@ Is called *after* the in-memory UTXO cache is flushed.
 
 Arguments passed:
 1. Time it took to flush the cache microseconds as `int64`
-2. Flush state mode as `uint32`. It's an enumerator class with values `0`
-   (`NONE`), `1` (`IF_NEEDED`), `2` (`PERIODIC`), `3` (`ALWAYS`)
+2. Flush state mode as `uint32`. It's an enumerator class with values
+   `0` (`NONE`), `1` (`IF_NEEDED`), `2` (`PERIODIC`), `3` (`FORCE_FLUSH`),
 3. Cache size (number of coins) before the flush as `uint64`
 4. Cache memory usage in bytes as `uint64`
 5. If pruning caused the flush as `bool`