The motivation of this change is to allow undo pages for temporary tables
to be marked free as often as possible, so that we can avoid buf_pool.LRU
eviction (and writes) of undo pages that contain data that is
no longer needed. For temporary tables, no MVCC or purge of history
is needed, and reusing cached undo log pages might not help that much.

It is possible that this may cause some performance regression due to
more frequent allocation and freeing of undo log pages, but I only
measured a performance improvement.

trx_write_serialisation_history(): Never cache temporary undo log pages.

trx_undo_reuse_cached(): Assert that the rollback segment is persistent.

trx_undo_assign_low(): Add template<bool is_temp>. Never invoke
trx_undo_reuse_cached() for temporary tables.

Tested by: Matthias Leich