* implement random access index in file_store where keys are u64 and values are serialised to a constant size
* cleanup: move all webgraph store writes into store_writer
* add a 'ConstIterableStore' that can store items on disk without needing to interleave headers in the case that all items can be serialized to a constant number of bytes known up front
* change edges file format to make edges for a given node iterable.
this allows us to only load a subset of the edges for a node in the future
* compress webgraph labels in blocks of 128
* ability to limit number of edges returned by webgraph
* sort edges in webgraph store by the host rank of the opposite node
