DatabasePlugin: add method GetUpdateStamp()

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

src/db/ProxyDatabasePlugin.cxx

@@ -71,6 +71,11 @@ public:
 			      DatabaseStats &stats,
 			      Error &error) const override;
 
+	virtual time_t GetUpdateStamp() const override {
+		// TODO: implement
+		return 0;
+	}
+
 private:
 	bool Configure(const config_param &param, Error &error);
 

src/db/SimpleDatabasePlugin.hxx

@@ -26,8 +26,6 @@
 
 #include <cassert>
 
-#include <time.h>
-
 struct Directory;
 
 class SimpleDatabase : public Database {
@@ -55,11 +53,6 @@ public:
 
 	bool Save(Error &error);
 
-	gcc_pure
-	time_t GetLastModified() const {
-		return mtime;
-	}
-
 	static Database *Create(const config_param &param,
 				Error &error);
 
@@ -85,6 +78,10 @@ public:
 			      DatabaseStats &stats,
 			      Error &error) const override;
 
+	virtual time_t GetUpdateStamp() const override {
+		return mtime;
+	}
+
 protected:
 	bool Configure(const config_param &param, Error &error);