Improve carrier SLA pricing API

As a business user I would like a more flexible shipping pricing solution where I can create/import multiple pricing options for carrier SLA.
I expect pricing to take account of country code and state code to determine basic price for shipment.

TD:

• reuse existing product pricing framework
• form SKU as [Carrier SLA GUID][Country Code][State Code] where country code and state code are optional
• quantity tier can be used as coefficient for basic price (e.g. if pricing is weight based then quantity can indicate weight thus allowing to treat tiers as weight levels)
• the applicability of SLA for the current cart now will be determined by existence of valid price. Since price has to be a positive number 'Free' shipping will now only be available via promotions engine, which also makes API more consistent