vinyl: cancel reader and writer threads on shutdown

Currently, vinyl won't shutdown until all reader and writer threads
gracefully complete all their pending requests, which may take a while,
especially for writer threads that may happen to be doing compaction at
the time. This is annoying - there's absolutely no reason to delay
termination in such a case. Let's forcefully cancel all threads, like we
do in case of relay threads.

This should fix sporadic vinyl/replica_quota test hang.

Closes #3949