relay: Make the max pending tasks per CPU a consensus parameter

relay: Make the max pending tasks per CPU a consensus parameter

Until now, there was this magic number (64) used as the maximum number
of tasks a CPU worker can take at once.

This commit makes it a consensus parameter so our future selves can
think of a better value depending on network conditions.

Part of #40704

Signed-off-by: David Goulet <dgoulet@torproject.org>