Drop no-longer-needed buffers during ALTER DATABASE SET TABLESPACE.

The previous coding assumed that we could just let buffers for the
database's old tablespace age out of the buffer arena naturally.
The folly of that is exposed by bug #11867 from Marc Munro: the user could
later move the database back to its original tablespace, after which any
still-surviving buffers would match lookups again and appear to contain
valid data.  But they'd be missing any changes applied while the database
was in the new tablespace.

This has been broken since ALTER SET TABLESPACE was introduced, so
back-patch to all supported branches.

src/backend/commands/dbcommands.c

@@ -1126,6 +1126,23 @@ movedb(const char *dbname, const char *tblspcname)
 	RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT
 					  | CHECKPOINT_FLUSH_ALL);
 
+	/*
+	 * Now drop all buffers holding data of the target database; they should
+	 * no longer be dirty so DropDatabaseBuffers is safe.
+	 *
+	 * It might seem that we could just let these buffers age out of shared
+	 * buffers naturally, since they should not get referenced anymore.  The
+	 * problem with that is that if the user later moves the database back to
+	 * its original tablespace, any still-surviving buffers would appear to
+	 * contain valid data again --- but they'd be missing any changes made in
+	 * the database while it was in the new tablespace.  In any case, freeing
+	 * buffers that should never be used again seems worth the cycles.
+	 *
+	 * Note: it'd be sufficient to get rid of buffers matching db_id and
+	 * src_tblspcoid, but bufmgr.c presently provides no API for that.
+	 */
+	DropDatabaseBuffers(db_id);
+
 	/*
 	 * Check for existence of files in the target directory, i.e., objects of
 	 * this database that are already in the target tablespace.  We can't