lncfg+scripts: use configurable global lock for postgres backends

Replace hardcoded WithGlobalLock assignment with configurable
options wallet postgres backends. Also add the WithGlobalLock
option to the channeldb table for postgres backends.

Defaults:
- channeldb: false (allow concurrent access)
- wallet: true (maintain safe single-writer behavior)

Users can now override these defaults via:
- db.postgres.channeldb-with-global-lock
- db.postgres.walletdb-with-global-lock

This gives operators flexibility while maintaining safe defaults
until full native SQL migration is complete.

Moreover exclude db.postgres.walletdb-with-global-lock check
in the sample config file script. We cannot easily check the
correct default because we set it later in the LND startup
sequence so we exclude it.

lncfg/db.go

@@ -115,7 +115,15 @@ func DefaultDB() *DB {
 		},
 		Postgres: &sqldb.PostgresConfig{
 			MaxConnections: defaultPostgresMaxConnections,
-			QueryConfig:    *sqldb.DefaultPostgresConfig(),
+			// Normally we don't use a global lock for channeldb
+			// access, but if a user encounters huge concurrency
+			// issues, they can enable this to use a global lock.
+			ChannelDBWithGlobalLock: false,
+			// Default to true to maintain safe single-writer
+			// behavior until the wallet subsystem is upgraded to
+			// a native sql schema.
+			WalletDBWithGlobalLock: true,
+			QueryConfig:            *sqldb.DefaultPostgresConfig(),
 		},
 		Sqlite: &sqldb.SqliteConfig{
 			MaxConnections: defaultSqliteMaxConnections,
@@ -400,9 +408,15 @@ func (db *DB) GetBackends(ctx context.Context, chanDBPath,
 		// users to native SQL.
 		postgresConfig := GetPostgresConfigKVDB(db.Postgres)
 
+		// Create a separate config for channeldb with the global lock
+		// setting if configured.
+		postgresConfigChannelDB := GetPostgresConfigKVDB(db.Postgres)
+		postgresConfigChannelDB.WithGlobalLock = db.Postgres.
+			ChannelDBWithGlobalLock
+
 		postgresBackend, err := kvdb.Open(
 			kvdb.PostgresBackendName, ctx,
-			postgresConfig, NSChannelDB,
+			postgresConfigChannelDB, NSChannelDB,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("error opening postgres graph "+
@@ -450,14 +464,11 @@ func (db *DB) GetBackends(ctx context.Context, chanDBPath,
 		}
 		closeFuncs[NSTowerServerDB] = postgresTowerServerBackend.Close
 
-		// The wallet subsystem is still not robust enough to run it
-		// without a single writer in postgres therefore we create a
-		// new config with the global lock enabled.
-		//
-		// NOTE: This is a temporary measure and should be removed as
-		// soon as the wallet code is more robust.
+		// Create a separate config for wallet with the global lock
+		// setting if configured.
 		postgresConfigWalletDB := GetPostgresConfigKVDB(db.Postgres)
-		postgresConfigWalletDB.WithGlobalLock = true
+		postgresConfigWalletDB.WithGlobalLock = db.Postgres.
+			WalletDBWithGlobalLock
 
 		postgresWalletBackend, err := kvdb.Open(
 			kvdb.PostgresBackendName, ctx,