replace now() with os:timestamp() in some places

The now() call requires global locking to ensure monotonicity, so replace
some calls to now() with calls to os:timestamp() where monotonicity isn't
required. There are probably other such calls in other modules that can be
similarly replaced.

Calculate size of data sent only when stats requires it.
update cookie documentation

Documentation for cookies used the old yaws_api:setcookie functions, which
are deprecated. Change the docs to use yaws_api:set_cookie/3 instead. Also
add a little more detail to cookies.yaws regarding how
yaws_api:set_cookie/3 is used and what it returns.