Starting with commit baf276e6d4 (MDEV-19229)
the parameter innodb_undo_tablespaces can be increased from its
previous default value 0 while allowing an upgrade from old databases.
We will change the default setting to innodb_undo_tablespaces=3
so that the space occupied by possible bursts of undo log records
can be reclaimed after SET GLOBAL innodb_undo_log_truncate=ON.
We will not enable innodb_undo_log_truncate by default, because it
causes some observable performance degradation.
Special thanks to Thirunarayanan Balathandayuthapani for diagnosing
and fixing a number of bugs related to this new default setting.
Tested by: Matthias Leich, Axel Schwenke, Vladislav Vaintroub
(with both values of innodb_undo_log_truncate)
