server: use decred's rpcclient for BTC

[amass01]

That's the second part of and closes #841, #938 was the first part and it
migrated the client to use decred's v6 context aware rpcclient.

[JoeGruffins]

I had the simnet tests fail a few times while testing this, but not sure if related to these changes.

[amass01]

@chappjc had to adjust: 8d43164 after rebasing onto master.

[JoeGruffins]

Does cloneCfg.FeeEstimator/estimateFee's signature work as estimateFee func(context.Context, RawRequester) (uint64, error)? I'm not sure but maybe this would avoid a lot of exporting?

[amass01]

@JoeGruffins Thanks for the suggestion, was sure it's not idiomatic.

[chappjc]

You could also define a wrapper for callers that are not context-enabled so that it just uses the btc.Backend's subsystem shutdown context

func (rc *RPCClient) RawRequest(string, []json.RawMessage) (json.RawMessage, error){
    return rc.Requester.RawRequest(rc.ctx, method, params)
}

So that BCH can do

func estimateFee(node btc.RPCClient) (uint64, error) {
	resp, err := node.RawRequest("estimatefee", nil)

instead of

func estimateFee(node btc.RPCClient) (uint64, error) {
	resp, err := node.Requester.RawRequest(node.Ctx, "estimatefee", nil)

[amass01]

@chappjc will do 💯

[amass01]

@JoeGruffins @chappjc updated

[JoeGruffins]

Works for me.