Follow-up to thread-per-server monitoring

CDRIVER-3625 creates independent server monitors for multi-threaded and makes mongoc_topology_scanner_t exclusively single threaded.

This tracks additional cleanup to better separate the single threaded and multi threaded scanning logic.

• Multi-threaded server monitors should not reach into the mongoc_topology_scanner_t (currently that happens for SSL opts and a custom stream initiator)
• Remove the no longer needed mongoc_interrupt_t