multi: add dcr wallet client.

[dnldd]

Work towards #36.

[buck54321]

Consider renaming this PR to "wallet: add contract script functions" so we can get it in. I don't want to block wallet development while you're working on DEX API pieces.

[dnldd]

will be adding to the initial with new commits.

[buck54321]

Definitely on the right track, though the Client will need quite a bit of work still. I'm going to push the BTC wallet client in a couple days, and there will be substantial changes needed for them to have a common functional interface. I can see two ways of handling this.

1. Let's get this PR reviewed and merged, and I'll adapt the Client separately.

2. Leave this PR idle for a couple days and we can look at the two wallets side-by-side.

I'm leaning towards #1, but I'm not crazy about merging the Client in it's current state. What do you think, @chappjc?

[chappjc]

@buck54321 Either way is OK with me, although leaving this opened until the btc wallet client is up might benefit both PRs, although that's likely to result in more iterations to get them both merged. Your call.

[buck54321]

I think we should merge it after review. Let's not fret too much about tests, since they would likely need substantial rewrites after the interface is reworked anyway.

[buck54321]

Lots of work needed, but some good stuff here. I'll bring it all together with the BTC wallet. Ideally we would never merge something in this state of completion, but since I'm taking over this work and this code will help, I'm thinking we can do it this time.