add _timezone DSN query parameter

Josh Bleecher Snyderrequested to merge
josharian/sqlite:add-timezone into master

_timezone accepts an IANA timezone name, such as "UTC", and applies it to both reads and writes.

For writes, time.Time values are converted to the target timezone before formatting.

For reads, timezone-less strings are interpreted as being in the target timezone.

Supporting configuration at the DB driver level helps avoid industrial accidents when working with SQLite datetime strings, which lack time zones.

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