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>

contrib/tracing/README.md

@@ -246,10 +246,10 @@ $ python3 contrib/tracing/log_utxocache_flush.py $(pidof bitcoind)
 
 ```
 Logging utxocache flushes. Ctrl-C to end...
-Duration (µs)   Mode       Coins Count     Memory Usage    Prune
-730451          IF_NEEDED  22990           3323.54 kB      True
-637657          ALWAYS     122320          17124.80 kB     False
-81349           ALWAYS     0               1383.49 kB      False
+Duration (µs)   Mode         Coins Count     Memory Usage    Flush for Prune
+2556340         IF_NEEDED    2899141         394844.34 kB    False
+2005788         FORCE_FLUSH  2238117         310189.68 kB    False
+2685            FORCE_FLUSH  0               262.24 kB       False
 ```
 
 ### log_utxos.bt