sss_ptr_hash: internal refactoring
    
    sss_ptr_hash code was refactored:
     - got rid of a "spy" to make logic cleaner
     - table got destructor to wipe its content
     - described some usage limitation in the documentation
    
    And resolves: https://pagure.io/SSSD/sssd/issue/4135
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>