Add a memory limit to AskVM

Apart from operations limit it would be great to assert memory usage and limit it to a value set in a config file.

Start by locating AskVM and the configuration handling in the repository, since the issue names no files or tests. Determine how operation limits are enforced and where memory usage can be asserted. Done means a configured memory limit is enforced and covered by tests.

Written by the indexing model from the issue text.