* extract all network code to legacy submodule
* update references to legacy proto
* skeleton of futures-based protocol
* refactor skeleton to use background task
* rename communication_for to build_table_router
* implement internal message types for validation network
* basic ParachainNetwork and TableRouter implementations
* add some module docs
* remove exit-future from validation
* hack: adapt legacy protocol to lack of exit-future
* generalize RegisteredMessageValidator somewhat
* instantiate and teardown table routers
* clean up RouterInner drop logic
* implement most of the statement import loop
* implement statement loop in async/await
* remove unneeded TODO
* most of the collation skeleton
* send session keys and validator roles
* also send role after status
* use config in startup
* point TODO to issue
* fix test compilation
Robert Habermeier