Make the collectorID independent of the desc order.

This is actually the intended behavior, and (as a nice side effect)
makes things cheaper to calculate.

Also, introduce a separator character to avoid hash collisions
(like label values {"ab","c"} vs {"a", "bc"}).

Apply the same principles to signature.go.

Change-Id: I607db544f278ed89684fe5fa11abdbc3e03d3061