trx_commit_complete_for_mysql(): Remove some conditions.
We will rely on trx_t::commit_lsn.

trx_t::must_flush_log_later: Remove. trx_commit_complete_for_mysql()
can simply check for trx_t::flush_log_later.

trx_t::commit_in_memory(): Set commit_lsn=0 if the log was written.

trx_flush_log_if_needed_low(): Renamed to trx_flush_log_if_needed().
Assert that innodb_flush_log_at_trx_commit!=0 was checked by
the caller and that the transaction is not in XA PREPARE state.
Unconditionally flush the log for data dictionary transactions,
to ensure the correct processing of ddl_recovery.log.

trx_write_serialisation_history(): Move some code from
trx_purge_add_undo_to_history().

trx_prepare(): Invoke log_write_up_to() directly if needed.

innobase_commit_ordered_2(): Simplify some conditions.
A read-write transaction will always carry nonzero trx_t::id.
Let us unconditionally reset mysql_log_file_name, flush_log_later
after trx_t::commit() was invoked.