The encodings are thread-safe and already used in such a manner in other places.
This isn't done where Byte Order Mark output is suppressed, since Encoding.UTF8 is constructed to output the BOM.
This is necessary so that code in HttpServer can use framework facilities such as the thread watchdog for monitoring purposes.
Doing this shuffle meant that MainServer was moved into OpenSim/Framework/Servers
Also had to make OpenSim.Framework.Console rely on OpenSim.Framework rather than the other way around since it in turn relies on HttpServer
MainConsole and some new interfaces had to be moved into OpenSim/Framework to allow this. This can be reverted if parts of OpenSim.Framework stop relying on console presence (cheifly RegionInfo)