Cache tz -> *time.Location results

[tysonmote]

Description

snowflakeRows.Next calls getCurrentLocation for every row, which means that the driver calls time.LoadLocation (which loads the TZ database) for every single row in the results of a query. Instead, we should only fetch it once, which we can do on the Config object.

In a simple application that runs queries and fetches a large number of rows, getCurrentLocation accounts for 65% of our overall CPU usage:

Checklist

• [x] Code compiles correctly
• [x] Run make fmt to fix inconsistent formats
• [x] Run make lint to get lint errors and fix all of them
• [ ] Created tests which fail without the change (if possible)
• [ ] All tests passing
• [ ] Extended the README / documentation, if necessary

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