Support for more than 64 nodes

[lundgren87]

The current implementation of the Pyxis directory uses two unsigned long (assumed to be 64 bit) to represent the sharers and writers of each page, in which each node is represented by one bit. This puts a hard limit of nodes in the system at 64 nodes. All fixed-width integer types impose the same kind of limit on the amount of nodes possible.

Ideally I believe this should be handled inside a Pyxis class with a proper interface, or at least a wrapper class that does not expose the internals of the storage. A quicker solution might be to use vector<bool> as each bool is represented by one bit. vector<bool> has some drawbacks and is not guaranteed to be contiguous for all sizes, so this should be investigated first.