flex/http: factor some source our of server.hpp

Move the Connection class to the server.cpp implementation file
since it's definition doesn't need to be in the header. Move
StaticallyBufferedStream into its own stream implementation file.
flex/http: make request handler state checking assert()-based
flex/impls/server_io: add correct move constructor/destructor to SocketStreamIO
flex/http: flush connection after writing 100-expect response
flex/ifaces: add virtual destructors to type-erased interfaces
flex/ifaces: remove unused socket.hpp file