While removing the globals from background.go it also made sense to clean up the use of dcrd RPC clients. Previously two seperate clients were maintained, one for making RPC calls and one for receiving notifications. These clients have been unified.
- Rename all instances to "shutdownCtx" to be really explicit. This context is special in that it can be closed at any time without warning, so it should be obvious to the reader.
- Don't use shutdownCtx in RPC clients. Clients should not stop working immediately when shutdown is signalled, they need to keep working while the process is shutting down.
* Remove global cache variable.
Rather than maintaining cached data in a global variable, instantiate a cache struct and keep it in the `Server` struct.
* Store net params in RPC clients.
This means net params only need to be supplied once at startup, and also removes a global instance of net params in `background.go`.
Bring in some updates from the signal handling code in other projects:
- Rename `signals` to more descriptive `interruptSignals`.
- Add SIGHUP to unix shutdown signals. Rename `signalsigterm.go` to `signal_unix.go` accordingly.
- Include extra detail in "Already shutting down..." messages log lines.
- Pass a shutdown func into `webapi.go` rather than a channel. It's more obvious how to invoke a func, whereas a channel can be used in multiple ways.
