Set max amount of epochs for state cache to retain states

[povi]

Set max amount of epochs for state cache to retain states, in order to avoid high memory usage during long non-finalization periods.

This is related to #59

[sauliusgrigaitis]

This makes deep reorgs complicated. I think it makes sense to benchmark higher amounts of epochs in memory and set a value that makes deeps reorgs easier.

[povi]

I've increased the max amount of states to retain in state cache to 64. In non-finality benchmark, this increased the memory usage from 3.6 GiB to 6.4 GiB, which is still reasonable amount of memory used. This should make deep-reorg events easier for client to handle.

[povi]

I'm also thinking about adding a CLI argument that overrides this max epochs amount if needed