remove dependency on std::iostream from core

It is still required for the iostream transport and the basic
(iostream) logging policy.
Adds a stub transport policy

This will be used in the minimal config to avoid bringing in iostream
dependencies. It will also be used to document the minimal interface
for writing a custom transport policy.
Add minimal_client and minimal_server configs references #338
remove references to unused iostream header from asio transport
added a note about breaking changes in the logging policy API
Merge branch 'master' into core_dependencies

Conflicts:
	changelog.md
make version introduced notes less specific
Merge core requirements feature branch