feat: miner: max prove commit sector batch size

[rvagg]

The lack of a batch max size for the NI-PoRep method @ #1537 came up during an audit yesterday given that one is applied to precommit batches and it was noted that there's no max applied on prove_commit_sectors3, although there is one applied on aggregate proof size.

This may not be necessary here since I suppose it's already limited by the precommit limit, but I wanted to check whether that's correct or not. Perhaps the limitation imposed in this PR should only be added to #1537 for the reason documented @

builtin-actors/runtime/src/runtime/policy.rs

Lines 241 to 242 in 2899d1b

	// 32 sectors per epoch would support a single miner onboarding 1EiB of 32GiB sectors in 1 year.
	pub const PRE_COMMIT_SECTOR_BATCH_MAX_SIZE: usize = 256;

[anorth]

What we actually want to do is remove the pre-commit max batch size. #1268

[rvagg]

ok, that's good to know; we can ignore this now but we should take this into account in #1537