MinPieceSize for SetAsk is lost on restart

[hannahhoward]

Describe the bug
If you set a MinPieceSize via lotus-miner storage-deals set-ask and then restart the miner, it resets to the default value

To Reproduce
Steps to reproduce the behavior:

1. Run lotus-miner storage-deals set-ask --price 100000000000 --verified-price 100000000000 --min-piece-size 250MiB --max-piece-size 32GiB
2. Restart lotus-miner
3. Run lotus-miner storage-deals get-ask
4. See MinPieceSize is back to default value

Expected behavior
The value set should persist

Version (run lotus version):
master

Additional context

The line that causes this is here:
https://github.com/filecoin-project/lotus/blob/master/node/modules/storageminer.go#L399

But more importantly, when you call SetAsk on StoredAsk in the markets code, unless you pass a value for MinPieceSize, it's not copied over.

Proposed solution

SetAsk should retain any existing MinPieceSize/MaxPieceSize when called (i.e. in the if s.ask != nil clause)
Also the the code referenced above is problematic cause it will overwrite any MaxPieceSize. The proper solution I think should be to let NewStoredAsk take a var args StorageAskOption list -- if there is a value already on disk, this list is unused, but if the code here is run:
https://github.com/filecoin-project/go-fil-markets/blob/master/storagemarket/impl/storedask/storedask.go#L90,
Then pass the var args list to it.

Then go back and add the sector size to the call to NewStoredAsk.