Set a lower default MaxMemInQueues value

The default MaxMemInQueues value of 0.75*RAM assumes:

• there is one tor instance per machine, and
• MaxMemInQueues covers most queue memory.

If we instead assumed:

• two tor instances, and
• MaxMemInQueues covers half of queue memorym this would make Tor more resilient to attacks.

To do this, we should set MaxMemInQueues to 0.2*RAM, at least if we have a lot of RAM (like 4-8GB or more).