Skip cache slot when time-to-live is zero

If time-to-live is set to zero, we don't need to regenerate the cache
slots on every request. Instead, just skip the caching process and
immediately provide the dynamically generated version of the page.
Setting time-to-live to zero is useful when you want to disable caching
for certain pages.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
Clean up cache documentation.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>