When MDEV-6076 repurposed the field PAGE_MAX_TRX_ID, it was assumed
that the field always was 0 in the clustered index of old data files.
This was not the case in IMPORT TABLESPACE (introduced in MySQL 5.6
and MariaDB 10.0), which is writing the transaction ID to all index
pages, including clustered index pages.

This means that on a data file that was at some point of its life
IMPORTed to an InnoDB instance, MariaDB 10.2.4 or later could interpret
the transaction ID as a persistent AUTO_INCREMENT value.

This also means that future changes that repurpose PAGE_MAX_TRX_ID
in the clustered index may cause trouble with files that were imported
at some point of their life.

There is a separate minor issue that InnoDB is writing PAGE_MAX_TRX_ID
to every secondary index page, even though it is only needed on leaf
pages. From now on we will write PAGE_MAX_TRX_ID as 0 to non-leaf pages,
just to be able to keep stricter debug assertions.

btr_root_raise_and_insert(): Reset the PAGE_MAX_TRX_ID field on non-root
pages of the clustered index, and on the no-longer-leaf root page of
secondary indexes.

AbstractCallback::is_root_page(): Remove. Use page_is_root() instead.

PageConverter::update_index_page(): Reset the PAGE_MAX_TRX_ID to 0
on other pages than the clustered index root page or secondary index
leaf pages.