At startup, we load the outpoints to watch, *then* roll back 15
blocks.  If there were things in those blocks we wanted to watch, we
no longer do!

1. We load the utxoset into memory: everything in the utxoset table
   which has spendheight null.
2. We roll back 15 blocks to re-read.  Deleting a block from the
   database causes the utxo spentheights referring to it to be set
   to null.
3. We roll forward, but we didn't update the in-memory utxoset,
   so we're not watching those utxos which are spent.

The main symptom of this is that we spam peers with obsolete gossip
(if we get sent a channel announcement for a closed channel, we can
think it isn't spent yet).  But it could *also* mean we don't notice
onchain txs, if we restart at the wrong time!

Changelog-Fixed: lightningd: we could miss tx spends which happened in the past blocks when we restarted.
Signed-off-by: Rusty Russell <[email protected]>

We only do this once, and not on new nodes.

Signed-off-by: Rusty Russell <[email protected]>

I took the node from the original test which missed the UTXO, and
added some more blocks.

Signed-off-by: Rusty Russell <[email protected]>

Signed-off-by: Rusty Russell <[email protected]>