Increase our default/minimum dust limit to 354 sat/vbytes

330 sat/vbyte, the current value, is not sufficient to ensure a
future segwit script longer than 32 bytes meets the dust limit if
used for a shutdown script. Thus, we can either check the value
on shutdown or we can simply require segwit outputs and require a
dust value of no less than 354 sat/vbyte.

We swap the minimum dust value to 354 sat/vbyte here, requiring
segwit scripts in a future commit.

See lightning/bolts#905