Add some caching for password hash validation.

Add some caching for password hash validation.

Password hash functions must be expensive in order to be secure. But
if they have to be re-evaluated for every request, performance
suffers.

As a minimal remedy, cache the most recent result for every
connection. This gives a great performance boost if a web browser
does many requests on the same connection with the same
user+password.  In principle, this may keep the plain text password
around longer than before. But in practice, there won't be much
difference since user+password can already remain in some unused
data bucket for longer than the request duration.

A proper solution still needs to be found for connections from
proxies which may carry requests for many different users.

While it currently only requires the conn_rec, the new
ap_password_validate() function takes username and request_rec to
allow future extensions, like detection of brute-force attempts.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1427548 13f79535-47bb-0310-9956-ffa450edef68