Cannot use entire UID space from clients

For the record, I shortcutted this function in dgraph and then all UIDs were available to insert. This makes my application change from every insert needs to be an atomic insert - to, everything being a straight up insert to a specific UID and eventual consistency is cool so use ludicrous mode.

So, I think this use case is interesting and could be a huge performance increase for our application. At very least the bugs I listed above should be addressed. (allocating all uint64 uids returns Server not initialized, and allocating a large block or all UIDs makes the alpha unsure of how many UIDs have been leased)