to avoid expensive function calls until the refcount reaches 0

Signed-off-by: Stephan Bergmann <sbergman@redhat.com>, slightly changing it to
add a uno_type_sequence_destroy to uno/sequence2.h instead of a
uno_type_destructSequence to uno/data.h.

Change-Id: I3bbff3294f2b515fc3c68c4c6c1cb16829f5cc44