SHA256Double: helper to avoid manually calling SHA256 twice.

In fact, this is all we need.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
c++: add .gitignore.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Add --log-block-txids to log iblt-useful information to stdout.

To evaluate the use if IBLTs, we need to know the difference between
set of txs in a block, and the set of txs we know about.  This patch
produces messages like so:

block-txids: Block <blockid>
block-txids: unknown tx <txid>
 ...
block-txids: unused tx <txid>
 ...

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Use logging to look at txids which come in (debugging)

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