Check that users of `node` do not get bitten by IBD when starting a `bitcoind` node

[tcharding]

These code comments are in electrsd:

        // electrum will remain idle until bitcoind is not in IBD
        // bitcoind will remain in IBD if doesn't see a block from a long time, thus adding a block

[tcharding]

cc @RCasatta

[tcharding]

Note the first code comment is actually different, there is a typo on master at the moment. Depending on whether its been patched or not the line may read:

// electrum will remain idle until bitcoind is in IBD