Config build tags to build subsytems by default

[matheusd]

Most subsystems are meant to be built in the final production build but aren't currently due to use of build tags.

Upstream this is solved by producing release builds via make but we don't want to rely on that given our preferred build tool.

So we should switch build tags from positive (e.g. routerrpc) to negative (!routerrpc) so that by default the desired subsystems are included in a release build and you don't need anything other than `go build' to get a full fledged dcrlnd node.

[matheusd]

We might wanna consider whether to include walletrpc by default at this moment though, since dcrwallet has a better and more complete grpc server.

[fguisso]

Working on it

[fguisso]

done in #77