server/book: automatic book reallocation

[chappjc]

This hides the notion of capacity from Book consumers by creating a book with modestly-sized buy/sell priority queues instead of the mammoth defaults of before, and the underlying OrderPQs automatically reallocate into a larger queue when Inserts push the queue to capacity.

The queue increases by the larger of minCapIncrement (4096) or 1/8th of the current utilization (note that utilization is the same as capacity when realloc is triggered by Insert->Push).

The queue will also automatically reallocate to a smaller capacity when Extract*/Remove*/Pop reduces the utilization of the queue to a point where the ideal capacity is significantly below the current capacity, specifically savings > deallocThresh && savings > pq.capacity/3 where deallocThresh = 10 * minCapIncrement.

[buck54321]

Couple little questions, but looks good. Fewer levers and less duplication.