[Wilt Chamberlain] fix for dupe check_cols values in snapshot strategy

[drewbanin]

• Makes target_database optional in snapshots (I'm not sure if the way I implemented it was the best way to do this)
• Preprends the dbt_scd_id hash with the unique id of the row, avoiding hash conflicts

The existing behavior of the check_cols strategy could fail in a subtle way if the specified check_cols values for two rows were equivalent:

Given:

id	name	color
1	alice	red
2	bob	red

Assume that bob's color changes to blue:

id	name	color
1	alice	red
2	bob	blue

The generated snapshot query would invalidate any records where dbt_scd_id = md5('red'). This would inadvertently update alice as well as bob!

This PR includes the unique id in the hash, so the resulting snapshot would invalidate records where dbt_scd_id = md5('2' || 'red'), which is localized to just bob's record.