Graceful handling of MapFull error in slasher

[michaelsproul]

Description

The slasher (#1567) is capable of setting a max-db-size for LMDB, but it's currently quite useless in that once the max size is reached, the slasher never recovers from the MapFull errors that get returned. I made a tweak that I thought would allow pruning to run (and clear some space), but pruning itself fails with a MapFull error, even though it only does gets and deletes 🤔

[michaelsproul]

I think this is still an issue, based on the fact that slasher DB sizes are steadily increasing.

[michaelsproul]

Closing in favour of #2286, as I think our disk usage issues are not due to a bug in pruning