Checking this in middleware not only removes duplicated code from each of the handlers, but also makes things more efficient. Previously, dcrd/dcrwallet/database clients were initialized (and could possibly cause errors) **before** checking if the VSP is closed. Now the check happens before those other things.
Also bundled with some improvements to setaltsignaddr_test.go which I implemented along the way.
* Remove helper func getCommitmentAddress.
This function is only used in one place so removing it does not introduce any duplication. Removing the func also removes the need for errors.Is comparison, which will be very useful in upcoming changes.
* Rename apiError to ErrorCode.
* Don't use inline type for API errors.
* Export webapi errors.
* Export webapi request/responses.
* Add types module for API requests/responses/errors
Periodic database backups are no longer started automatically in database.Open(), and the backup written by database.Close() can now be disabled.
Only vspd itself requires backups, they are not useful for test code or future upcoming tools such as vote-validator.
This removes all of the global loggers from the project and replaces them with loggers which are instantiated in vspd.go and passed down as params.
To support this, the background package was removed. It only contained one file so it was a bit pointless anyway. It only existed so background tasks could have their own named logger.
`CanTicketVote` doesn't really fit into the RPC client code as it is more of a business logic function. Moving it into the webapi package is more appropriate.
Running as sub-tests has the benefit of automatically logging the test name, no need to include it in failure messages manually. It even works if the test panics.
