Further performance improvements by avoiding allocation.

Avoid allocation of QByteArray on the write side of Property change
serialization.

Cache the newed packets, this helps because the memory of the QStrings
in the packets can be reused and doesn't need a new allocation and
we save the packet allocation.

Change-Id: I3848013c222d89508b7011a60f964883c01d7dd6
Reviewed-by: Brett Stottlemyer <bstottle@ford.com>