mergeFiles in Merge/BlockC doesn't free Mempool on error

[amosr]

mergeFiles doesn't clean up the last Mempool if there's an error.
It creates a memory pool, then every few hundred iterations it creates a new one and copies data over.
It's hard to bracket because the memory pools aren't lexically scoped.

The solution is to store the current iteration state in an IORef or something, so that the bracket end function can read the IORef and free the most recent Mempool.

[amosr]

https://github.com/ambiata/zebra/pull/30/files/bc5ff076cb4cc6f05eee3bc5e17e398edbee1ecf#diff-526302b6954165c6d34dfdf7c5a0fa65R62