`jrpc-echod`, `walletd`: implement `help <method-name>`

[msgilligan]

[liamgilligan]

Implementation plan is as follows: In EchoJsonRpcService, we'll define a "helpMessage" record, which will store the short and long help strings for a given method.

We will also initialize a map, mapping method name strings to a corresponding "helpMessage" record.

In the help method, we will simply retrieve the respective help message, defaulting to the help message for help + logic for when no/invalid method names are given.

[msgilligan]

Implementation plan is as follows: In EchoJsonRpcService, we'll define a "helpMessage" record, which will store the short and long help strings for a given

Note that the issue calls for adding help to both echod and wallets. You could use the same approach in both, of course. But maybe later there might be some common code to put in the jsonrpc library module...

[liamgilligan]

See #300

[msgilligan]

See #300

300 implements help for echod but not for walletd. Let's leave this issue open until we have it implemented for walletd too.