With Sylvan, we are allowed to call deref(bdd) in parallel.
Use this to free BDDs that were found by the GC in a separate thread,
such that we do not need to check for to-be-freed BDDs
on every operation in the main thread.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13106 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c