s3:locking: convert share_mode_lock.c to generate_unique_u64()

s3:locking: convert share_mode_lock.c to generate_unique_u64()

Instead of a sequence number that gets incremented we just
need a value that's not reused.

The is a similar change like the commit before at the g_lock.c
layer.

I expect a similar performance improvement here, but
I don't know a specific benchmark test to check.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>