rpcserver: Convert to make use of new btcjson.

This commit converts the RPC server over to use the new features available
in the latest version of btcjson and improve a few things along the way.
This following summarizes the changes:

- All btcjson imports have been updated to the latest package version
- The help has been significantly improved
  - Invoking help with no command specified now provides an alphabetized
    list of all supported commands along with one-line usage
  - The help for each command is automatically generated and provides much
    more explicit information such as the type of each parameter, whether
    or not it's optional or required, etc
  - The websocket-specific commands are now provided when accessing the
    help when connected via websockets
  - Help has been added for all websocket-specific commands and is only
    accessible when connected via websockets
- The error returns and handling of both the standard and websocket
  handlers has been made consistent
- All RPC errors have been converted to the new RPCError type
- Various variables have been renamed for consistency
- Several RPC errors have been improved
- The commands that are marked as unimplemented have been moved into the
  separate map where they belong
- Several comments have been improved
- An unnecessary check has been removed from the createrawtransaction
  handler
- The command parsing has been restructured a bit to pave the way for
  JSON-RPC 2.0 batching support