db/Interface: add attribute "plugin"

The new method IsPlugin() replaces the "is_simple" flag.

src/db/plugins/LazyDatabase.cxx

@@ -23,6 +23,9 @@
 
 #include <assert.h>
 
+LazyDatabase::LazyDatabase(Database *_db)
+	:Database(_db->GetPlugin()), db(_db), open(false) {}
+
 LazyDatabase::~LazyDatabase()
 {
 	assert(!open);

src/db/plugins/LazyDatabase.hxx

@@ -35,8 +35,7 @@ class LazyDatabase final : public Database {
 
 public:
 	gcc_nonnull_all
-	LazyDatabase(Database *_db)
-		:db(_db), open(false) {}
+	LazyDatabase(Database *_db);
 
 	virtual ~LazyDatabase();
 

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -91,7 +91,8 @@ class ProxyDatabase final : public Database, SocketMonitor, IdleMonitor {
 
 public:
 	ProxyDatabase(EventLoop &_loop, DatabaseListener &_listener)
-		:SocketMonitor(_loop), IdleMonitor(_loop),
+		:Database(proxy_db_plugin),
+		 SocketMonitor(_loop), IdleMonitor(_loop),
 		 listener(_listener) {}
 
 	static Database *Create(EventLoop &loop, DatabaseListener &listener,

src/db/plugins/SimpleDatabasePlugin.cxx

@@ -40,6 +40,10 @@
 
 static constexpr Domain simple_db_domain("simple_db");
 
+inline SimpleDatabase::SimpleDatabase()
+	:Database(simple_db_plugin),
+	 path(AllocatedPath::Null()) {}
+
 Database *
 SimpleDatabase::Create(gcc_unused EventLoop &loop,
 		       gcc_unused DatabaseListener &listener,

src/db/plugins/SimpleDatabasePlugin.hxx

@@ -50,8 +50,7 @@ class SimpleDatabase : public Database {
 	mutable unsigned borrowed_song_count;
 #endif
 
-	SimpleDatabase()
-		:path(AllocatedPath::Null()) {}
+	SimpleDatabase();
 
 public:
 	gcc_pure

src/db/plugins/UpnpDatabasePlugin.cxx

@@ -75,6 +75,8 @@ class UpnpDatabase : public Database {
 	UPnPDeviceDirectory *discovery;
 
 public:
+	UpnpDatabase():Database(upnp_db_plugin) {}
+
 	static Database *Create(EventLoop &loop, DatabaseListener &listener,
 				const config_param &param,
 				Error &error);