Rebuilding ledger - but first 50K gone

[martinsumner]

There is a leveled store, with lots of objects, but all of the first 50K objects added have now been updated/deleted, and the first journal file has been compacted.

This means that the starting SQN (i.e. the first SQN in the first file) is 50000+ not 1.

In this case, if the ledger is cleared, when there is an attempt to rebuild the ledger, the loader will loop ... but whilst pushing to the penciller empty data every 1000 SQN between 1 and 50000. This means that the penciller memory is pushed empty lists via push_mem - not expected!

When the penciller's in-memory level zero is full - it will try and write a level zero file from an empty list - and crash.

[martinsumner]

#390