cm-message: use tp_cm_message_get_sender() internally
use tp_message_dup_part() instead of tp_message_peek()
message: change string getter to _dup_

When switching to GVariant we won't be able to guarantee the lifetime of the
returned string so best to dup them.
logger: use tp_message_get_pending_message_id()
logger: use tp_message_dup_part()
echo-message-parts: use tp_message_dup_part()
stop using tp_message_peek() in tests
stop using tp_message_peek() internally
remove tp_message_peek()
client-factory: sink the newly created GVariant if props is NULL