SDPMessage, FlowCombiner and ParseContext have specific functions
available to perform copying, freeing and (un)ref'ing. Calling them
directly on versions where they are supported prevents us from going
through GType machinery and locks that end up the same functions in the
end.
They can actually be shared with multiple threads at the same time
safely as all functions requiring an immutable reference are
thread-safe.
OTOH TypeFind can't be shared safely between different threads as not
all implementations of the TypeFind struct are thread-safe.
