Events database GC

[rvagg]

Tie event lifetime to blockstore lifetime. Make this configurable but if you have a splitstore enabled then events shouldn't persist if you don't have the blocks available for them.

Needs some consideration though: eth_getLogs and GetActorEventsRaw I think should still work on old epochs even if you don't have the blocks, maybe that's desirable? We could make this configurable to either have a config option with "number of epochs to store" or "follow splitstore".