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

trx_commit_complete_for_mysql(): Remove some conditions.
We will rely on trx->commit_lsn.

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.