Send block number instead of the epoch seed hash

[chfast]

1. The mining software has to search for the epoch number anyway, because that's needed to create light cache and DAG - the sizes of light cache and full DAG are functions of the epoch number.
2. Epoch / block number has 4 bytes max, seed hash 32 bytes.
3. Block number is better than epoch number, because:
   • you can estimate when the epoch change will occur,
   • it's more informational for users (e.g. you can log the current block number),
   • it's easier to explain in the spec what the block number is than what the epoch number is.

I recommend creating version 1.1.0 of the spec with this change.