UndorecApplier::assign_rec(): Remove. We will pass the undo record to
UndorecApplier::apply_undo_rec(). There is no need to copy the
undo record, because nothing else can write to the undo log pages
that belong to an active or incomplete transaction.

trx_t::apply_log(): Buffer-fix the undo page across mini-transaction
boundary in order to avoid repeated page lookups.

Reviewed by: Vladislav Lesin