forknet: support arbitrary shard layouts (#12938)

We'd like to use forknet with more shards than we have currently on
mainnet, or just any arbitrary shard layout. But it won't work to just
change the shard layout in the genesis file, since we need to rewrite
everything in the tries. So this PR implements support for an optional
custom shard layout when generating the state.

To do this, we use the existing `StorageMutator` that keeps a list of
updates per shard, and we change it to be aware of which shards belong
to the source shard layout and which belong to the target. We still have
one thread per source shard, but now it writes state to shards that
possibly don't exist in the shard layout.

This also requires writing a new bandwidth scheduler state in each
target shard that doesn't exist in the source shard layout