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>