sqldb+lncfg: consolidate SQLite default constants

Move the SQLite default constants (max connections, busy timeout) to
sqldb/config.go as the single source of truth and export them. Remove
the duplicate definitions from lncfg/db.go and reference the sqldb
constants instead.

lncfg/db.go

@@ -40,9 +40,6 @@ const (
 	DefaultBatchCommitInterval = 500 * time.Millisecond
 
 	defaultPostgresMaxConnections = 50
-	defaultSqliteMaxConnections   = 2
-
-	defaultSqliteBusyTimeout = 5 * time.Second
 
 	// NSChannelDB is the namespace name that we use for the combined graph
 	// and channel state DB.
@@ -126,8 +123,8 @@ func DefaultDB() *DB {
 			QueryConfig:            *sqldb.DefaultPostgresConfig(),
 		},
 		Sqlite: &sqldb.SqliteConfig{
-			MaxConnections: defaultSqliteMaxConnections,
+			MaxConnections: sqldb.DefaultSqliteMaxConns,
-			BusyTimeout:    defaultSqliteBusyTimeout,
+			BusyTimeout:    sqldb.DefaultSqliteBusyTimeout,
 			QueryConfig:    *sqldb.DefaultSQLiteConfig(),
 		},
 		UseNativeSQL:           false,

sqldb/config.go

@@ -32,15 +32,15 @@ type SqliteConfig struct {
 }
 
 const (
-	// defaultSqliteMaxConns is the default number of maximum open
+	// DefaultSqliteMaxConns is the default number of maximum open
 	// connections for SQLite. SQLite only supports a single writer, so a
 	// low default reduces contention on the busy_timeout and limits
 	// resource usage, especially on mobile.
-	defaultSqliteMaxConns = 2
+	DefaultSqliteMaxConns = 2
 
-	// defaultBusyTimeoutMs is the default busy_timeout value in
+	// DefaultSqliteBusyTimeout is the default busy_timeout value used
-	// milliseconds, used when no BusyTimeout is configured.
+	// when no BusyTimeout is configured.
-	defaultBusyTimeoutMs = 5000
+	DefaultSqliteBusyTimeout = 5 * time.Second
 )
 
 // busyTimeoutMs returns the busy_timeout value in milliseconds. If
@@ -50,7 +50,7 @@ func (s *SqliteConfig) busyTimeoutMs() int64 {
 		return s.BusyTimeout.Milliseconds()
 	}
 
-	return defaultBusyTimeoutMs
+	return DefaultSqliteBusyTimeout.Milliseconds()
 }
 
 // MaxConns returns the effective maximum number of open connections. If
@@ -62,7 +62,7 @@ func (s *SqliteConfig) MaxConns() int {
 		return s.MaxConnections
 	}
 
-	return defaultSqliteMaxConns
+	return DefaultSqliteMaxConns
 }
 
 // Validate checks that the SqliteConfig values are valid.