Internally cache return values of wp_salt(). Always run the filter. Big...

Internally cache return values of wp_salt(). Always run the filter. Big performance gains on a pageload that generates hundreds of nonces. see #19599.

git-svn-id: https://core.svn.wordpress.org/trunk@20135 1a063a9b-81f0-0310-95a4-ce76da25c4cd