peer: keep current commit txs, anchor state, channel funding and their sig.

This lets us implement accept_pkt_anchor().

Also had to predeclare sha256 in commit_tx.h, revealed by the new
includes.

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

commit_tx.h

@@ -6,6 +6,7 @@
 
 struct channel_state;
 struct sha256_double;
+struct sha256;
 struct pubkey;
 struct rel_locktime;