sqldb+lncfg: use SQLite-appropriate default for max connections

Both the sqldb and kvdb SQLite layers were not using a sensible default
for MaxConnections. The sqldb store used defaultMaxConns (25) which is
meant for Postgres, and the kvdb path passed 0 (unlimited) to
sqlbase.Init when unconfigured.

Add a MaxConns() method on SqliteConfig that returns the configured
value or defaults to 2, appropriate for SQLite's single-writer model.
Use it in both sqldb/sqlite.go and lncfg/db.go so both layers share
the same default.

sqldb/sqlite.go

@@ -136,8 +136,8 @@ func NewSqliteStore(cfg *SqliteConfig, dbPath string) (*SqliteStore, error) {
 			err)
 	}
 
-	db.SetMaxOpenConns(defaultMaxConns)
-	db.SetMaxIdleConns(defaultMaxConns)
+	db.SetMaxOpenConns(cfg.MaxConns())
+	db.SetMaxIdleConns(cfg.MaxConns())
 	db.SetConnMaxLifetime(connIdleLifetime)
 	queries := sqlc.New(db)