Combine appdisc and servicedisc packages

[i-hate-nicknames]

Packages app/appdisc and servicedisc both make use of service discovery (SD). Appdisc implements advertising apps (i.e. writing) (but also visor as a public service) in SD, while servicedisc implements fetching entities from SD (i.e. reading) and also common logic between these two packages. HttpClient structure in servicedisc is tailored for writes (it requires host address), which makes it confusing to use for the reads.

I propose to combine them into a single package, possible with subpackages for reading and writing interaction with SD.

[jdknives]

See comment here also.