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GH-38795: [Go] Fix race GetToTimeFunc for Timestamp (#38797)



### Rationale for this change
Adding RWMutex to protect `loc` in `TimestampType` and fix the race condition.

### Are these changes tested?
Yes, a unit test is added which is covered by the CI which runs with `-race`.

### Are there any user-facing changes?
Copying `TimestampType` will now be problematic and linters will show it as an error. In theory this shouldn't be a problem as most uses of TimestampType should be utilizing pointers to it rather than the value directly.

* Closes: #38795

Lead-authored-by: Matt Topol <zotthewizard@gmail.com>
Co-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Ben Harkins <60872452+benibus@users.noreply.github.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>