GenerateRandomID returns a unique, 64-character ID consisting of a-z, 0-9.
It guarantees that the ID, when truncated (TruncateID) does not consist
of numbers only, so that the truncated ID can be used as hostname for
containers.
TruncateID returns a shorthand version of a string identifier for convenience.
A collision with other shorthands is very unlikely, but possible.
In case of a collision a lookup with TruncIndex.Get() will fail, and the caller
will need to use a longer prefix, or the full-length Id.