The problem was that the maximum value of the transaction_prealloc_size
session system variable was ULONG_MAX which meant that it was possible
to cause the server to allocate excessive amounts of memory.

This patch fixes the problem by reducing the maxmimum value of
transaction_prealloc_size and transaction_alloc_block_size down
to 128K.

Note that transactions will still be able to allocate more than
128K if needed, this patch just reduces the amount that can be
preallocated - as well as the maximum size of the incremental
allocation blocks.