xds: Use tracing GC in XdsDepManager

Reference counting doesn't release cycles, so swap to a tracing garbage
collector. This greatly simplifies the code as well, as diffing is no
longer necessary. (If vanilla reference counting was used, diffing
wouldn't have been necessary either as you just increment all the new
objects and decrement the old ones. But that doesn't work when use a set
instead of an integer.)

Author: ejona86