LDAP: Refactor account info handler into a tevent request
    
    The sdap account handler was a function with its own private callback
    that directly called the back end handlers. This patch refactors the
    handler into a new tevent request that the current sdap handler calls.
    
    This refactoring would allow the caller to specify a custom sdap
    connection for use by the handler and optionally retry the same request
    with another connection inside a single per-provider handler.
    
    No functional changes are present in this patch.