trx_commit_cleanup(): Clean up any temporary undo log.
Replaces trx_undo_commit_cleanup() and trx_undo_seg_free().

trx_write_serialisation_history(): Commit the mini-transaction.
Do not touch temporary undo logs. Assume that a persistent rollback
segment has been assigned.

trx_serialise(): Merged into trx_write_serialisation_history().

trx_t::commit_low(): Correct some comments and assertions.

trx_t::commit_persist(): Only invoke commit_low() on a mini-transaction
if the persistent state needs to change.