An alternative to the unity build, the classic build compiles each
translation unit individually. This adds more modules to the classic build:
* Remove unity header app.h
* Add missing includes as needed
* Remove obsolete NodeStore backend code
* Add app/, core/, crypto/, json/, net/, overlay/, peerfinder/ to classic build
The MultiSocket class implements a socket that handshakes in multiple
protocols including SSL and PROXY. Unfortunately the way it type-erases the
handlers and buffers is incompatible with boost::asio coroutines. To pave the
way for coroutines this is part of a larger set of changes that roll back the
usage of MultiSocket to older code, and some custom implementations that use
templates. The custom implementations are more simple since they use
coroutines. Removing MultiSocket will make many other classes and source files
unused, a big win for trimming down the codebase size.
