Create u64 as unique identifier for a RIB

[density215]

We are already creating a random 12 char ascii alphanumeric string to use as the name of the directory that stores the LSM trees for a RIB. We don't have a unique identifier for the RIB itself, though. The idea is that we create a randomized u64 and then use the hex representation of its octets as the directory name. Also, we should check for collisions with directories of former instanced of Rotonda (currently we don't).