MINOR: task: give the scheduler a bit more flexibility in the runqueue size

MINOR: task: give the scheduler a bit more flexibility in the runqueue size

Instead of setting a hard-limit on runqueue-depth and keeping it short
to maintain fairness, let's allow the scheduler to automatically cut
the existing one in two equal halves if its size is between the configured
size and its double. This will allow to increase the default value while
keeping a low latency.

(cherry picked from commit 1691ba3693d4b11cc85aed606d39ffbd23d8533f)
Signed-off-by: Willy Tarreau <w@1wt.eu>