Have a hash in the unique URL instead of username

Follow up from #2792091: Add unique URL copy link to the product node page

The unique URL for the product is in format :

http://yoursite.com/p/<username>/<product_node_id>

Change the format to:

http://yoursite.com/p/<userhash>/<product_node_id>

Proposed solution:

• Create a table users_hash storing a user_id and its hash. Populate the data on hook_install().
• On new user creation, save the hash using hook_user_insert().
• Remove the data from the table on user deletion on hook_user_delete().
• In commerce_commission_set_referral_product() query for user id belonging to the hash instead of the username.