Listen to bitcoind with ZeroMQ

[jotapea]

Use ZeroMQ to listen for wallet transactions and new blocks, replacing lightning's native event subscriptions.

Needed to continue with #317

[jotapea]

Go with v6? Using v5 for now, as it is still suggested in the docs.

EDIT: Using v6 after all, is typescript ready among other benefits.

[nicolasburtey]

@jotapea ZeroMQ is already active on our bitcoind I believe. this is also how lnd is getting all its transactions.

[jotapea]

@nicolasburtey Thanks for the info (also updated the title).