We introduce an EXTENDED log record for appending an undo log record
to an undo log page. This is equivalent to the MLOG_UNDO_INSERT record
that was removed in commit f802c989,
only using more compact encoding.

mtr_t::log_write(): Fix a bug that affects longer log
record writes in the !same_page && !have_offset case.
Similar code is already implemented for the have_offset code path.
The bug was unobservable before we started to write longer
EXTENDED records. All !have_offset records (FREE_PAGE, INIT_PAGE,
EXTENDED) that were written so far are short, and we never write
RESERVED or OPTION records.

mtr_t::undo_append(): Write an UNDO_APPEND record.

log_phys_t::undo_append(): Apply an UNDO_APPEND record.

trx_undo_page_set_next_prev_and_add(),
trx_undo_page_report_modify(),
trx_undo_page_report_rename():
Invoke mtr_t::undo_append() instead of emitting WRITE records.