SGTools; Allow marking keys as transient to avoid hash collisions.

When we record a new object to SG, we get an error if the key+clid hashes to an already-existing value. Allow marking a SG string key as `transient' by prefixing it with an underscore. In this case, we don't fail on a hash collision, but instead assign this key+clid an unused hash. This is only allowed for transient SG keys, since the hash used will not correspond to the result of the hash function for these cases, which could be confusing for persistent data.