rpcclient: implement createwallet with functional options

[onyb]

Rebased on #1645, to avoid having to resolve merge conflicts later. This PR only implements createwallet as a first-step towards functional options.

Background

I find the current way of dealing with optional parameters in RPC commands very hard to maintain, and also clunky from a user's perspective. We implement one rpcclient method per optional parameter, with long-winding names, closely following the order of the parameters!

This has been previously discussed a bit in #1589.

Current style

We have multiple methods to choose from, depending upon the options you want to use. In the example below, it is impossible to specify just watchOnly without specifying count. 😞

ListTransactions(account string)
ListTransactionsCount(account string, count int)
ListTransactionsCountFrom(account string, count int, from int)
ListTransactionsCountFromWatchOnly(account string, count int, from int, watchOnly bool)

Proposed style

ListTransactions(
        "myaccount",
        WithListTransactionsWatchOnly(),  // btcjson.Bool(true) if specified, default (nil) otherwise
        WithListTransactionsCount(100),   // Use options in any order you like!
)

The above is known as functional options pattern, which is widely used in the Go community. Dave Cheney has written about it here. Here's another article I really liked.

The API is made possible using the following variadic signature:

func (c *Client) ListTransactions(account string, opts ...ListTransactionsOpt) FutureListTransactionsResult

[coveralls]

Pull Request Test Coverage Report for Build 298236815

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 10 of 48 (20.83%) changed or added relevant lines in 2 files are covered.
• 83 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.1%) to 53.371%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/wallet.go	0	38	0.0%

Files with Coverage Reduction	New Missed Lines	%
integration/rpctest/blockgen.go	2	85.61%
database/ffldb/blockio.go	4	92.62%
peer/peer.go	11	75.47%
btcjson/walletsvrcmds.go	16	96.09%
rpcclient/wallet.go	50	0.0%

Totals
Change from base Build 289529546:	-0.1%
Covered Lines:	20787
Relevant Lines:	38948

---

💛 - Coveralls

[jcvernaleo]

High level, I'm cool with this style. Going to do a real review now though.

[jcvernaleo]

Looks good to me. I like the new style. Will give it a few days in case anyone else wants to look and then i'll merge.

[jakesylvestre]

I like this. Must cleaner than what we have now and less tedious to implement than the builder strategy

[jakesylvestre]

Looks good to me! The only thing I might add is we should clarify some of our style guidelines here (https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md) at some point

[onyb]

@jakesyl Indeed, we should document this. I'll do a separate PR to update the guidelines.

If you can, please feel free to merge the PR.