Moved PriceTree struct into maker.go

PriceTree is renamed to priceTree so that it is private and located
where it is intended to be used.
Red/Black tree implementation of a guidstore

Now tested and benchmarked. As a first blush it looks like it is
achieving more than 5 million writes per second on my atom
chromebook.

The caveat to that is that if we need to allocate a new node
latency will take a (currently unmeasured) hit. However, I expect to
be able to predict the range of expected guid and we can preallocate
the guidstore's internal structure to eliminate this problem.