Simplify semantics of ChainStateFlushed callback

Previously, ChainStateFlushed would fire either if a full flush
completed (which can happen due to memory limits, forced flush, or
on its own DATABASE_WRITE_INTERVAL timer) *or* on a
ChainStateFlushed-specific DATABASE_WRITE_INTERVAL timer. This is
both less clear for clients (as there are no guarantees about a
flush having actually happened prior to the call), and reults in
extra flushes not clearly intended by the code. We drop the second
case, providing a strong guarantee without removing the periodit
timer-based flushing.

src/validationinterface.h

@@ -99,6 +99,17 @@ protected:
     /**
      * Notifies listeners of the new active block chain on-disk.
      *
+     * Prior to this callback, any updates are not guaranteed to persist on disk
+     * (ie clients need to handle shutdown/restart safety by being able to
+     * understand when some updates were lost due to unclean shutdown).
+     *
+     * When this callback is invoked, the validation changes done by any prior
+     * callback are guaranteed to exist on disk and survive a restart, including
+     * an unclean shutdown.
+     *
+     * Provides a locator describing the best chain, which is likely useful for
+     * storing current state on disk in client DBs.
+     *
      * Called on a background thread.
      */
     virtual void ChainStateFlushed(const CBlockLocator &locator) {}