Add get_fork_version and add_ideal_fork_version to core so
cryptonote_protocol does not have to need the Blockchain
class directly, as it's not in its dependencies, and add
those to the fake core classes in tests too.
- fix wrong block being used when a new block is received between
a node elaying a fluffy block and sending a new fluffy block
with txes a peer did not have
- misc a neverending ping pong requesting the same missing txids
when a new block is received in the meantime, causing the top
block to not be the one we need
- send the original fluffy block message block height when sending
a new fluffy block, not the current top height, which might
have been updated since
- avoid sending back the whole block blob when asking for txes,
send only the hash instead
- plus misc cleanup and additional debugging logs
