Inside InnoDB, each mini-transaction that generates any redo log records
will acquire log_sys->mutex during mtr_t::commit() in order to copy the
records into the global log_sys->buf for writing into the redo log file.

For single-row transactions, this incurs quite a bit of overhead.
We would use two mini-transactions for writing a record into a
freshly updated undo log page. (Only if the undo record will
not fit in that page, then we will have to commit and restart
the mini-transaction.)

trx_undo_assign(): Assign undo log for a persistent transaction,
or return the already assigned one.

trx_undo_assign_low(): Assign undo log for an operation on a
persistent or temporary table.

trx_undo_create(), trx_undo_reuse_cached(): Remove redundant parameters.
Merge the logic from trx_undo_mark_as_dict_operation().