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`.
This allows both tspend and treasury policies to be set by clients on a per-ticket basis. Preferences can be set when initially registering a ticket with `/payfee`, and can be later updated using `/setvotechoices`.
Any requests which alter treasury/tspend policy will be stored in the database using the existing accountability system.
**Note:** This does not include consistency checking, it will need to be added later when dcrwallet has an RPC to retrieve policies in batches.
