Use less memory for MaxMemInQueues for machines with more than 8 GB of RAM.

This patch changes the algorithm of compute_real_max_mem_in_queues() to
use 0.4 * RAM iff the system has more than or equal to 8 GB of RAM, but
will continue to use the old value of 0.75 * RAM if the system have less
than * GB of RAM available.

This patch also adds tests for compute_real_max_mem_in_queues().

See: https://bugs.torproject.org/24782