A fix for gh-617 - add a separate fiber key for the current user.

Use cached user credentials for global access checks and store
them in a separate fiber key.

This fixes gh-617 and speeds up privilege checks.

@todo: we need to add a linked list of all user sessions
to the user cache, as well as all user setuid functions.
These lists could be used to update privilege cache,
get/update per-user statistics, disconnect sessions
of a dropped user.