Replace glue_cache hashtable with direct link in rdatasetheader

Instead of having a global hashtable with a global rwlock for the GLUE cache, move the glue_list directly into rdatasetheader and use Userspace-RCU to update the pointer when the glue_list is empty.

Additionally, the cached glue_lists needs to be stored in the RBTDB version for early cleaning, otherwise the circular dependencies between nodes and glue_lists will prevent nodes to be ever cleaned up.

Closes #4045 (closed)