block: consider a block "known" if we know sufficient previous blocks.

We currently insist on knowing the contents of *all* previous blocks,
but that's overkill.  Instead of a boolean, keep a counter, and compare
it against the most we need to know to mine.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
recv_block: don't get ancient transactions.

Now we don't need all txs, don't get them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
recv_block: Don't log UNKNOWN_PREV errors during sync.

They're expected.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
generating: take care never to log { or } characters.

The fast-path in pettycoin-query introduced in 994a7facdca22b63b33cef7948b35e81fe5cf0df
won't work if we do this.  So hex-ize the nonce.

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