connection: hide mutability
connection: hide mutability

The mutability of the underlying constructs have nothing to do with the
mutability of the connection itself. For example, sending a message does
not change the connection itself, just the underlying OS primitives. To
fix this, wrap the members of Connection in RefCell to hide this
behavior.

Also, sending a message takes the ownership of the message.

srwalter/dbus-bytestream,srwalter/dbus-bytestream