dns: dlz_bind9 reference count logging

dns: dlz_bind9 reference count logging

dlz_bind9 has to count the number of times the plugin is 'created' by bind's
plugin manager so it doesn't repeat setup.  Logging doesn't reflect this
reference counting logic properly and so messages like "samba_dlz: shutdown"
can, confusingly, come up when the database connection has not actually been
severed.  This patch adds the necessary logging.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13655
Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>