Optimize PTree::deserializeSelf

There are a couple of significant improvements that could be made:

i) Read the attribute names directly out of the message buffer, rather than into StringAttrs. (StringBuffers would likely to be better0.
ii) Allocate all the attributes in one go, and avoid lots of setProp calls. (Would need to walk the buffer to count the number before starting. (It can also assume there is no previous value to free etc.)

This came up as a hotspot in some of the roxie query loading code (accessing the serialized graph). I suspect it would also improve the dali->component deserialization.