Virtual tables are kept in a thread_local registry for deduplication
purposes. The problem is that thread_local variables are destroyed late,
possibly after the schema registry and the reactor are destroyed.
Currently this isn't a problem, but after a seastar change to
destroy the reactor after termination [1], things break.
Fix by moving the registry to system_keyspace. system_keyspace was chosen
since it was the birthplace of virtual tables.
Pimpl is used to avoid increasing dependencies.
[1] 101b245ed7
2853f79f96