Do not persist all intermediary block proofs to disk

[Nashtare]

Since #96, we have the ability to prove a sequence of blocks in one go. This however writes all generated block proofs to disk, which is not needed.
More specifically, say we have checkpoint height $N$ and we generate blocks $N+1$ to $M$, then every subsequent proof $\pi_t$ will be attesting validity of all previous block proofs $pi_{N < i < t}$. As such, only the latter is really useful and needs to be persisted.