system.db.beginTransaction(database, isolationLevel, timeout)
String database - The name of the database connection to create a transaction in. Use "" for the project's default connection.
Integer/Constant isolationLevel - The transaction isolation level to use. Use one of the four constants: system.db.READ_COMMITTED, system.db.READ_UNCOMMITTED, system.db.REPEATABLE_READ, or system.db.SERIALIZABLE
Long timeout - The amount of time, in milliseconds, that this connection is allowed to remain open without being used. Timeout counter is reset any time a query or call is executed against the transaction, or when committed or rolled-back.
String - The new transaction ID. You'll use this ID as the "tx" argument for all other calls to have them execute against this transaction.