Add time and potentially memory constraints to queries

[floriankramer]

This is a feature that came up a couple of times already but was never implemented so far, so I thought adding an issue might help it getting some traction.
Given that #165 might have failed due to memory limitations adding a memory limit to queries could be useful to prevent the systems oom killer from activating due to expensive queries.

[floriankramer]

One possibility for adding time constraints would be using pthread_cancel on the std::thread::native_handle to kill the thread.

[niklas88]

I think @joka921 has done some investigation into this using a futures mechanism.

[hannahbast]

For quite some time now, QLever has a timeout (which can be set on a per-query basis), as well as a memory limit (which can be set via the -m option when starting the server).