* 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.
