s3:registry: hash the list of subkeys in the regsubkey_ctr

s3:registry: hash the list of subkeys in the regsubkey_ctr

This removes many loops over all the arrays
(from regsubkey_ctr_key_exists) and thus
reduces "net conf drop" from 1m55 to 48seconds
and "net conf import" from 1m55 to 58 seconds
for 2000 shares on my box.

Michael