pure virtual function call

[vldm]

Problem

Some tests failed with

pure virtual method called
terminate called without an active exception

This error originates from facebook/rocksdb#649
I also found that parity fixed it paritytech/substrate#4929 by joining runtime threadpool threads.

Proposed Solution

Looks like our problem is related, because it arives when tokio::spawn called on local cluster creation. So we need a way to join this thread before runtime stops.