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

btcjson/v2/btcjson/chainsvrcmds.go

@@ -417,7 +417,7 @@ func NewGetRawTransactionCmd(txHash string, verbose *int) *GetRawTransactionCmd
 // GetTxOutCmd defines the gettxout JSON-RPC command.
 type GetTxOutCmd struct {
 	Txid           string
-	Vout           int
+	Vout           uint32
 	IncludeMempool *bool `jsonrpcdefault:"true"`
 }
 
@@ -426,7 +426,7 @@ type GetTxOutCmd struct {
 //
 // The parameters which are pointers indicate they are optional.  Passing nil
 // for optional parameters will use the default value.
-func NewGetTxOutCmd(txHash string, vout int, includeMempool *bool) *GetTxOutCmd {
+func NewGetTxOutCmd(txHash string, vout uint32, includeMempool *bool) *GetTxOutCmd {
 	return &GetTxOutCmd{
 		Txid:           txHash,
 		Vout:           vout,