Potential deadlock in ImpalaServer::ExpireQueries()

IMPALA-12602 introduces a way to unregister a query from a session when idle_query_timeout is reached. However, it also includes logic in ExpireQueries() that could cause a deadlock by trying to get the SessionState::lock while also holding query_expiration_lock_. This violates the lock order defined in impala-server.h and could potentially result in a deadlock.

For example, it can have a deadlock with SetInFlight(), which may try to get the query_expiration_lock_ while holding SessionState::lock.