* Add test to replicate issue 459
Nothing actually crashes due to the issue - but looking at the logs there is the polarised stats associated with the issue. When merging into L3, you would normally expect to merge into 4 files - but actually we see FileCounter occasionally spiking.
* Add partial merge support
There is a `max_mergebelow` size which can be a positive integer, or infinity. It defaults to 32.
If a merge from Level N covers less than `max_mergebelow` files in level N + 1 - the merge will proceesd as before. If it has >= `max_mergebelow`, the merge will be curtailed when `max_mergebelow div 2` files have been created at that level. The remainder for Level N will then be written, as well as for Level N + 1 up to the next whole file that has no yet been touched by the merge.
The backlog that prompted the merge will still exist - as the files in Level N have not been changed. However, it is likely the next file picked will not be the same one, and will in probability have a lower number of files to merge (as the average is =< 8).
This will stop progress from being halted by long merge jobs, as they will exit out in a safe way after partial completion. In the case where the majority of files covered do not require a merge, then those files will be skipped the next time the remainder file is picked up for merge at Level N
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