Concurrent access to the same query module had a race condition on the
pointer that was used to handle the custom memory management. With this
commit, a mapping has been added to keep information about what
thread used the pointer to handle the memory resources. This should be
fine since the respected query executions are running on a dedicated
thread. Access to the mapping itself is threadsafe. A simple RAII
wrapper for the mapping container has also been added for simpler
client-side use.
