Allow miner/worker to restart sealing using the cache rather than wiping it and starting from scratch

[askender]

stores/local.go:391 /*/cache/s-t0**-**

if miner/worker quit and restart, cache will all be removed and redo precommit1. It waste a lot of time. Does it can be improved?

[f8-ptrk]

i think it has to be like that

#antidisestablishatarianism

[jennijuju]

@askender How do you think it can be improved?

[lanzafame]

When restarting, the miner should detect that the cached sector is present and continue from that point instead of wiping the cache sector and starting PC1 all over again. I am sure there are some reasons why this isn't currently done, but it would be nice to see this fixed.

[SgtCoin]

Archiving for future reference due to age.