Merge #16688: log: Add validation interface logging

f9abf4ab6d Add logging for CValidationInterface events (Jeffrey Czyz)
6edebacb21 Refactor FormatStateMessage for clarity (Jeffrey Czyz)
72f3227c83 Format CValidationState properly in all cases (Jeffrey Czyz)
428ac70095 Add VALIDATION to BCLog::LogFlags (Jeffrey Czyz)

Pull request description:

  Add logging of `CValidationInterface` callbacks using a new `VALIDATIONINTERFACE` log flag (see #12994). A separate flag is desirable as the logging can be noisy and thus may need to be disabled without affecting other logging.

  This could help debug issues where there may be race conditions at play, such as #12978.

ACKs for top commit:
  jnewbery:
    ACK f9abf4ab6d
  hebasto:
    ACK f9abf4ab6d
  ariard:
    ACK f9abf4a, only changes since 0cadb12 are replacing log indication `VALIDATIONINTERFACE` by `VALIDATION` and avoiding a forward declaration with a new include
  ryanofsky:
    Code review ACK f9abf4ab6d. Just suggested changes since last review (thanks!)

Tree-SHA512: 3e0f6e2c8951cf46fbad3ff440971d95d526df2a52a2e4d6452a82785c63d53accfdabae66b0b30e2fe0b00737f8d5cb717edbad1460b63acb11a72c8f5d4236

src/logging.h

@@ -54,6 +54,7 @@ namespace BCLog {
         COINDB      = (1 << 18),
         QT          = (1 << 19),
         LEVELDB     = (1 << 20),
+        VALIDATION  = (1 << 21),
         ALL         = ~(uint32_t)0,
     };