Major disk optimisation for non-full stores: When we create a new store, initialise the slot filters correctly, that is, set every slot to checked but not occupied. On stores which have been ~ 20% used, all the slots will have been filled anyway, but this is a big gain in disk I/O for fresh stores; slot filters were effectively not doing anything.
Add some tests for slot filters, including checking whether they return false positives, migration from a non-slotfilter store.
Test fewer blocks if extensive testing not enabled.
Delete the slot filter file if it exists when we have slot filters disabled.
Acceptable false positives depends on how many blocks we are testing.