This commit marks the point at which vspd will stop working against dcrd 1.6 and start to target master.
The dcrwallet RPC version has not changed, so vspd continues to work with dcrwallet 1.6 or master.
* Delay fee broadcast and adding tickets to wallets.
- Adds a `background` package which implements a dcrd notification handler. On each blockconnected notification, tickets with 6+ confirmations are marked confirmed, relevant fee transactions are broadcast, and any fees with 6+ confirmations have their tickets added to voting wallets.
- VSP fee is now an absolute value measured in DCR rather than a percentage. This simplifies the code and is more appropriate for an MVP. We can re-add percentage based fees later.
- Database code for tickets is now simplified to just "Insert/Update", rather than having functions for updating particular fields.
- Pay fee response no longer includes the fee tx hash, because we dont necessarily broadcast the fee tx straight away.
* Const for required confs
