Using a single select loop for background tasks removes a lot of
duplicated boilerplate code and helps to simplify shutdown logic. This
does reduce the amount of things which can run in parallel, but that
isn't of concern for vspd. The web server still runs in its own
goroutine so its responsiveness won't be affected.

The more verbose name was helpful in the past when some code was
handling more than one context, however that is no longer the case due
to refactoring so reverting to the more concise name makes sense.

vspd runs some tasks on startup - database consistency checks, catching
up with missed blocks, etc. If a shutdown is requested while these tasks
are running, vspd should stop immediately rather than continuing to
start the web API server and other background tasks.

Add a Context parameter to findSpentTickets so it can be canceled when a
shutdown is requested.

Check the status of the Context in blockConnected so it can be canceled
when a shutdown is requested.

Add a Context parameter to checkWalletConsistency so it can be canceled
when a shutdown is requested.