Removed INSERT ON DUPLICATE UPDATE approach

Use standard INSERT or UPDATE queries based on if the session has a non-zero/non-empty creation timestamp or not.
PHPDoc cleanup
Fixed sDB INSERT query result processing
Merge branch 'feature/session-rewrites' of code.shopp.net:core/shopp into 1.3.x
Merge branch '1.3.x' of code.shopp.net:core/shopp into 1.3.x