As of bitcoind 0.16.1, you can't send a single-input OP_RETURN output,
as you get 'tx-too-small'.
sendrawtx exit 26, gave error code: -26?error message:?tx-size-small (code 64)?'
So instead we use the minimum fee we can, but otherwise ignore it and
don't wait for it to be mined.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>