Protocol: change prev_txhashes to go back by powers of 2.

This means you're not scott free after 10 blocks: if you don't
know 1024 blocks back you'll be unable to mine in future.

We also parallelize update-mocks in the test makefile.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Clean up remaining references to 'prev_merkles'.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Check prev_txhashes when we resolve a new shard.

It's inefficient, but the code is simple.  We still don't send or handle
the appropriate complaint packet, but that's on my TODO list.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>